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Front Fork Oil discussion; filling amounts; hints.
This article applies to Pre-Airheads, Airheads, Classic K bikes

https://bmwmotorcycletech.info/front-fork-oils-amounts.htm 
section 54-10B
Copyright 2020, R. Fleischer

Discussion of oil types and characteristics:

The following section should be used with:  https://bmwmotorcycletech.info/viscosity.htm.

I prefer Spectro's fork oils and suspension fluids.  They are of good quality, have low stiction, wide temperature range (excellent viscosity index), & the viscosities can be depended upon.   Due to how specified, & lack of stiction fighters & other characteristics, you are better off with a real fork oil like Spectro's ....especially the full synthetic or part synthetic fork oils.   For fork oils & suspension fluids, the various manufacturer's do not agree on measuring viscosity; sometimes they don't do more than give some sort of approximate SAE grade value.  Viscosity measurements & temperatures are more accurate between manufacturer's for engine & gear oils; not so, apparently, for many suspension & fork oils. Because of these various things, and other reasons I won't bother to get into, I highly recommend you stay with one manufacturer, this is particularly so if you are trying different viscosity grades.

Back in early Airhead & K bikes days, the oil BMW used was "BMW red fork oil".  That oil was actually a military hydraulic oil, you could find its full specifications using a search engine for:  MIL-H-5606- (probably -E now).     Oil with a "5606" military specification can be depended-upon for viscosity (but not for some things you really want for your fork oil). To save you the trouble of finding the information, just the viscosity specifications for that oil are:
MINimum 4.9 Cst at 100 C (generally accepted as same as 210F).
MINimum 13.2 Cst at 40 C (100F).
Maximum 600 Cst at -40 C  ....too cold to bother about for motorcyclists (unless very crazy about cold weather riding!).
You can use those two viscosities, 4.9 Cst and 13.2 Cst, to compare (perhaps!), with other manufacturer's oils specs ...& with the information in the following article on this site:
https://bmwmotorcycletech.info/viscosity.htm

DO NOT DEPEND ON MANUFACTURER'S SAE NUMBERS ...USE MY CHARTED INFORMATION in that above LINK, for oil viscosity information!

Except for some GS models, all the BMW's (that this article applies to) need a rather thin oil, approximately SAE 4 (like that military oil, as above), so do not willy-nilly change the viscosity grade to much higher.   Only in the instance of heavy to very heavy loads (rider & passenger weights & luggage), or very aggressive performance, is an oil heavier than an actual SAE 7.5 weight of any benefit.  NEVER higher than SAE 10 for the non-GS models (some GS models specify use of different oils, SAE 10 and SAE 15, in right & left fork legs, respectfully).

UNDER NO CIRCUMSTANCES CHANGE OIL VISCOSITY TO COMPENSATE FOR FAILURE TO REMOVE AS MUCH STICTION IN THE FORKS AS POSSIBLE!

A medium-high viscosity index is generally a good thing for most riders for fork oils. However, a good argument can be made that in some circumstances ...a lower VI can have advantages.  One needs to THINK about how viscosity changes with heat (not just atmospheric, but the heat produced by considerably bumpy terrain, which affects viscosity, some less or more than others).  The viscosity index of an oil is simply how fast or how slow, with temperature rise and fall, the oil thickness changes.  There is an article on this website on actual viscosity and viscosity index values for oils; here is the link again, (it also has other oil information):  https://bmwmotorcycletech.info/viscosity.htm.  Once the VI is quite high, you might not notice any difference in performance by going higher in VI.  But, you MIGHT actually want a change ....with the forks not staying as stiff with a lot of fork movement.  HOWEVER, you may not want that ...it depends on YOUR riding preferences.    If you are an aggressive rider in VERY bumpy & probably constantly near extreme bumpy conditions, the fork oil will heat up a fair amount.  If your forks feel too soft under such conditions, but feel OK when starting out, then you may want to consider a high VI oil, even perhaps an oil that is a bit thicker (sometimes) to start with; and, for sure one that has anti-foaming characteristics.  If you think about what I have said here, you may just come to the conclusion that an overly-high (or overly-low) viscosity index is NOT necessarily what you may want.  Keep in mind that as the front end gets colder, everything, in general, has more friction (except maybe the tires!).

BMW Airheads forks from 1981 are especially sensitive to fork oil foaming.  I suggest you use a quality FORK or SUSPENSION fluid (oil).

Only a bit Nerdy:
Oils get thinner (viscosity decreases) as they heat up, and the reverse is true as oil temperature decreases. A reasonably decent argument can be made that it would be nice for some riders if their fork oils became somewhat thinner, proportionally, as they got colder.  Multigrade oils don't do this like you might think, they do help though, by changing less in viscosity over a temperature change whether higher or lower.   Still, some improvement ...in some respects ...will be possible by using multigrade oils.    Do consider stiction.  It is VERY important that forks, fender mounts, fork braces, etc., all be assembled very carefully to minimize stiction. 


Comments and chart of oil filling amounts:

NOTE:  These are pretty much standardized filling amounts and methods.  If you are an advanced-knowledge person with experience in suspension tuning, your methods may be different.

1.  In instances where I know the information, I have listed oil refilling amounts where different for a drain and a refill; and, a fork overhaul and a rebuild.  An overhaul/rebuild is where the fork parts are assembled with only a very small amount of oil, hardly many drops during the rebuilding; thus, very little volume of oil is left in the forks before you fill them with the proper oil amount, which will be somewhat larger than a drain/refill.    When you increase oil quantity beyond specifications (this is sometimes done when tuning the front end), the AIR above the oil level will DEcrease.  Air compresses, oil does not.  Thus, tuning is affected.  BMW's revised oil amounts is also shown in my charts.

2.  The /5/6/7 forks ...up to 1977....can be checked for level by inserting a thin rod (I use a full length welding rod).  The oil level should be 437 mm (17-1/4 inches) below the flat face of the fork top nut to the oil level.  Measure with the motorcycle on the center-stand, front forks fully extended (that only means the tire is not touching the ground).

3.  For the /5; /6; and /7 to 1980, with the forks fully extended, wheel off ground, the level is 50 mm above the damper.

4.  For the R45; R65 to 1985; R65LS; R80ST, the level is 35 mm above the damper,  +-15mm.

5.  For the /7 (1981 to 1984); R80GS, R80, R100RT & RS (1981-1984) you can insert a long rod & wiggle it, if needed, so the rod drops to the lowest point the rod will go to.   The level should be 300 mm from tip of rod to oil height shown.  That is 11-3/4 inches.

6.  Use a quality oil with no tendency towards foaming.

7.  The GS models have differing fork legs.  The RIGHT leg is for damping compression, the LEFT leg is for rebound damping.  I recommend using the same weight grade of oil in both legs.   The right leg oil will get dirtier much faster.  I suggest first trying a 10 weight oil for a rider, passenger, & loading. You could try Progressive Company springs, which hold up pretty good.  Use about 1 inch of preload.  Try 5cc or so less oil than specified.   You may want to experiment, & with some types of really heavy loadings, go to a higher viscosity oil, 15, and even try 20.

 

Drain & refill, cc (cubic centimeters)

Overhaul/rebuild; slight assembly oil only

PRE-Airheads:

 

 

1968-1969 U.S. models

265

280

pre-1968

150 to 170

 
     
Airheads:    
/5, /6, /7 to 1980

235 (265 is obsolete)

250 (280 is obsolete)

/7 from 1981, R80G/S, R80RT 1983-1984, R100 models from 1981

220

225

R45, R65 to 1985, R65LS, R80ST

190

195

R80 from 1985, R65 from 1986

300

 
R80RT from 1985, R100RS and RT from 1987 (1988?)

320

 
R80GS, R100GS, R100GSPD

Left (rebound) SAE 15, 410.

Right (compression) SAE 10, 440.

Left (rebound) SAE15, 470.

Right (compression) SAE 10, 470.

R80R, R100R, SHOWA forks

410

420

     
     
Classic K bikes:    
K1

380

400

K75 standard

325

 
K75 Showa, as on K75RT, etc

410

420

K75 SPORT; K100 SPORT (8 valve). Both have S stamped on the top aluminum plug

270-290

 
K100 (8 valve)

325

 
K100 (8 valve) RS, RT, LT, standard

355

 
K100 RS (16 valve)

380

400

K1100LT, K1100RS

Left 350; Right 400

400, 400

     
     

A chart of fork and suspension fluids information:
This table is sorted by real viscosity (centiStokes), not by nominal weight. Note that the nominal weight is ... um... not really in the same order as the real weight. Viscosity Index (VI) is how stable the viscosity is over a wide temperature range. Bigger numbers are better, but not as critical as you may think, unless, perhaps, being on the racetrack.   This is a mix of OLD information and later information, that is believed correct, but the manufacturer's have occasionally changed specifications, or changed the SAE rating for a specific oil.    In August, 2020, based on what is believed to be reliable information from someone who was in contact with Torco, Torco down-numbered some of their oils as far as (at least) SAE viscosity is concerned.  I decided to remove all mention of Torco from these charts.   Contact the company if you wish their published data.  I have NO recent lab testing on Torco products.  Here is a link to someone else's compilation of fork oil weights: 
http://mahonkin.com/~milktree/motorcycle/fork-oil.html
The charting is old, but contains much more than mine, and it has the Torco figures (out of date though)

Brand / weight / model nominal weight Reported cSt @40C (centiStokes) Reported cSt @100C (centiStokes) Reported Viscosity Index (VI) Recaclulated Viscosity Index (VI) Viscosity/ Temperature Coefficient (VTC) VT (cSt@40/cSt@100)
Red Line (like water, Clear)   5.50 2.30 344 343 0.58 2.39
Red Line (extra light, Blue)   9.80 4.00 402 403 0.59 2.45
Spectro, Golden (ultra light)   10.40 4.40 385 440 0.58 2.36
RockShox (2.5w Light cartridge (Torco 5) 2.5 10.75 4.00 325 340 0.63 2.69
               
Bel-Ray HVI (3 wt) 3 11.00 3.85 296 297 0.65 2.86
Kayaba (K2-S shock)   11.30 4.00 310 310 0.65 2.83
Showa Suspension Fluid SS-25 (2.5wt. Stock Shock, OEM) 2.5 12.50 3.90 239 239 0.69 3.21
Silkolene Pro RSF (2.5wt) 2.5 13.60 5.83 464 465 0.57 2.33
Maxima Racing Shock Fluid (light, 3wt) 3 14.00 5.20 378 370 0.63 2.69
               
PJ1 Cartridge Pro For Fluid (3.5 wt) 3.5 14.60 3.60   134 0.75 4.06
Castrol Fork Oil (10wt) 10 15.00 4.00 150 179 0.73 3.75
RaceTech (Motorex) (Light US1) 4.25 15.20 4.40 222 226 0.71 3.45
Penske (lightweight shock fluid)   15.50 6.30 434 434 0.59 2.46
Kayaba 01 (zero-One Fork)   15.60 3.45 150 96 0.78 4.52
Yamaha 01 Fork oil (Kayaba)   15.60 3.45 150 96 0.78 4.52
Showa Suspension Fluid SS-05 (Stock fork, OEM)   15.65 3.60 116 113 0.77 4.36
Maxima Bicycle Fork fluid (5wt) 5 15.90 3.51 150 98 0.78 5.53
Maxima Fork oil (5 wt) 5 15.90 3.51 150 98 0.78 4.53
Maxima Racing Fork fluid (85/150, 5 wt) 5 15.90 3.51 150 98 0.78 4.53
Motul Fork Oil (Very Light ) ??   16.00 3.50 100 95 0.78 4.57
Rockshox (52 Medium, Hydracoil) (Torco 7) 7 16.10 5.05 285 280 0.69 3.19
               
Motul Shock Oil (2.5-20)   16.10 6.20 400 403 0.61 2.60
showa Suspension Fluid SS-7 (5wt) (Apolo) 5 16.44 3.76 135 119 0.77 4.37
Spectro, Golden Spectro cartridge Fork Fluid (85/150, Very light)   16.90 3.82 150 119 0.77 4.42
Honda Pro HP Fork Oil (5wt) 5 17.00 3.80 140 115 0.78 4.47
Pro-Circuit PC-Oil   17.00 4.10   149 0.76 4.15
Bel-Ray Fork Oil (5 wt) 5 17.10 4.10 146 147 0.76 4.17
Kayaba G5 5 17.70 3.50 72 72 0.80 5.06
ELF Moto Fork oil SYN (2.5) 2.5 18.00 4.00 121 122 0.78 4.50
Motul Fork Oil, Comfort (Light 5) 5 18.20 4.00 124 119 0.78 4.55
Motul Fork Oil, Factory Line (Light 5) 5 18.30 4.20 138 138 0.77 4.36
Red Line (Light, Yellow)   18.40 7.10 407 409 0.61 2.59
Agip Fork (2.5 wt) 2.5 18.50 4.37 152 153 0.76 4.23
Bardahl (52 Fork Oil)   19.30 5.10 212 212 0.74 3.78
Bel-Ray HVI (5wt) 5 19.50 6.66 345 345 0.66 2.93
               
Motul Fork Oil (Light)??   20.00 6.00 260 280 0.70 3.33
Spectro, SX "400"   20.90 8.90 400 460 0.57 2.35
Maxima Racing Shock Fluid (Medium, 7wt) 7 21.00 7.30 349 358 0.65 2.88
Spectro Fork Oil (5wt) 5 21.60 4.40 119 114 0.80 4.91
Agip Fork (5wt) 5 22.00 5.20 175 178 0.76 4.23
Silkolene Fork Oil (5wt) 5 22.30 4.40 106 107 0.80 5.07
Motul Fork Oil, Factory Line (Medium, 5-10) 7.5 22.30 6.20 254 253 0.72 3.60
Elf Moto Fork OIL SYN (5) 5 23.30 4.80 130 129 0.79 4.85
PJ1 Fork Tuner (2.5wt) 2.5 26.10 4.90   111 0.81 5.33
Spectro, Golden Spectro Cartridge Fork Fluid (125/150, Very Light) (Marzzochi)   26.10 5.25 150 135 0.80 4.97
Spectro, Golden (Very Light)   26.40 9.90 400 397 0.63 2.67
Maxima Racing Fork Fluid (125/150), 7wt) 7 26.70 5.36 151 138 0.80 4.98
Silkolene Pro RSF (5wt) 5 26.70 9.46 372 372 0.65 2.82
PJ1 Fork Tuner (7.5wt) 7.5 27.50 5.10   113 0.81 5.39
Castrol Synthetic Fork Oil (5wt) 5 28.00 5.70 151 149 0.80 4.91
Ohlins (Shock)   28.70 9.40 340 340 0.67 3.05
Bel-Ray Fork Oil (7wt) 7 29.00 5.20 110 108 0.82 5.58
Maxima Racing Shock Fluid (Heavy, 10wt) 10 29.00 9.40 334 336 0.68 3.09
Ohlins (High Performance)   30.25 10.18 352 352 0.66 2.97
Red Line (Medium, Red)   30.40 10.80 369 374 0.64 2.81
Agip Fork (7.5wt) 7.5 31.00 6.40 163 165 0.79 4.84
PJ1 Fork Tuner (5wt) 5 31.20 5.40   106 0.83 5.78
Fox Shox (Torco RSF Medium)   31.82 9.57 300 309 0.70 3.32
               
Maxima Bicycle Fork Fluid (10wt) 10 32.00 6.28 151 151 0.80 5.10
Maxima Fork Oil (10wt) 10 32.00 6.28 151 151 0.80 5.10
Spectro Fork Oil (10wt) 10 32.80 5.60 111 107 0.83 5.86
RaceTech (Motorex) (Medium US2) 10 32.90 7.20 200 192 0.78 4.57
Bel-Ray Fork Oil (10wt) 10 33.00 5.90 123 123 0.82 5.59
RockShox (10wt) (Torco 15) 10 33.00 8.40 240 247 0.75 3.93
               
Shell Advance Fork 5 5 33.05 7.09 185 185 0.79 4.66
Kayaba G10S 10 33.20 5.70 111 111 0.83 5.82
Bel-Ray HVI (10wt) 10 33.50 10.00 306 308 0.70 3.35
PJ1 Fork Tuner (10wt) 10 33.90 5.70   106 0.83 5.95
Red Line D4 ATF   34.00 7.50 198 197 0.78 4.53
Mobil 1 Synthetic ATF   34.00 7.60 199 202 0.78 4.47
Silkolene Fork Oil (10wt) 10 34.90 7.05 169 169 0.80 4.95
Motul Fork Oil, Comfort (Medium 5-10) 7.5 35.30 6.00 115 114 0.83 5.88
Bardahl (10w Fork Oil) 10 35.50 6.90 158 158 0.81 5.14
Showa Suspension Fluid SS8 (10wt) (Apollo) 10 36.47 7.24 200 167 0.80 5.04
Silkolene Pro RSF (7.5wt) 7.5 37.00 12.00 322 339 0.68 3.08
Shell Advance Fork 10 10 38.97 7.44 161 161 0.81 5.24
Castrol Synthetic Fork Oil (10wt) 10 42.00 7.50 150 147 0.82 5.60
RockShox (15w Extra Heavy) (Torco 20) 15 42.80 10.40 250 245 0.76 4.12
               
Agip Fork (10wt) 10 45.00 8.30 164 162 0.82 5.42
Elf Moto Fork Oil SYN (10) 10 45.40 7.60 135 134 0.83 5.97
Spectro Fork Oil (15wt) 15 45.60 7.20 119 118 0.84 6.33
Repsol Moto Horquillas Fork Oil (10W20)   46.00 8.40 150 161 0.82 5.48
Maxima Bicycle Fork Fluid (15wt) 15 46.00 8.48 154 163 0.82 5.42
Maxima Fork Oil (15wt) 15 46.00 8.48 154 163 0.82 5.42
Maxima Racing Fork Fluid (235/150, 15wt) 15 46.00 8.48 154 163 0.82 5.42
Castrol Fork Oil (15wt) 15 46.50 8.30 150 155 0.82 5.60
ELF Moto Fork Oil (10wt) 10 47.00 8.30 153 153 0.82 5.66
Spectro, Golden (light)   47.27 16.76 235 370 0.65 2.82
Kayaba G15S 15 47.30 7.20 112 112 0.85 6.57
Silkolene Pro RSF (10wt) 10 47.36 13.69 303 303 0.71 3.46
Silkolene Fork Oil (15wt) 15 48.45 8.50 153 153 0.82 5.70
Bel-Ray HVI (15wt) 15 49.00 13.90 297 298 0.72 3.53
Bel-Ray Fork Oil (15wt) 15 53.00 7.50 103 103 0.86 7.07
Red Line High Temp ATF   53.90 10.00 172 176 0.81 5.39
PJ1 Fork Tuner (15wt) 15 55.30 8.70   133 0.84 6.36
Motul Fork Oil, Factory Line (Heavy, 15-20)   61.00 13.20 224 225 0.78 4.62
PJ1 Fork Tuner (20wt) 20 61.70 8.40   106 0.86 7.35
Spectro Fork Oil (20wt) 20 65.00 8.90 110 111 0.86 7.30
Maxima Bicycle Fork Fluid (20w) 20 65.00 12.90 201 204 0.80 5.04
Maxima Fork Oil (20w) 20 65.60 12.90 201 202 0.80 5.09
Red Line (Heavy)   66.80 16.00 256 256 0.76 4.18
Shell Advance Fork 15 15 67.00 10.37 120 143 0.85 6.46
Silkolene Fork Oil (20wt) 20 67.70 8.75 101 101 0.87 7.74
Castrol Fork Oil (20wt) 20 68.00 11.20 150 159 0.84 6.07
Repsol Moto Horquillas Fork Oil (15W30)   68.00 11.20 150 159 0.84 6.07
Agip Fork (15wt) 15 69.00 11.50 162 163 0.83 6.00
Elf Moto Fork Oil (15) 15 72.60 11.60 155 155 0.84 6.26
Bel-Ray Fork Oil (20wt) 20 73.50 9.50 106 107 0.87 7.74
Motul Fork Oil, Comfort (Heavy, 15-20)   77.90 10.10 111 112 0.87 7.71
               
Motorex Semi-Bath fork oil, 5W40   90.90 14.60 169 168 0.84 6.23
Silkolene Pro RSF (15wt) 15 92.50 19.50 235 233 0.79 4.77
ELF Moto Fork Oil (20) 20 99.00 14.60 150 153 0.85 6.78
Agip Fork (20wt) 20 104.00 15.03 155 151 0.86 6.92
Silkolene Fork Oil (30wt) 30 105.00 11.84 101 102 0.89 8.87
Bel-Ray Fork Oil (30wt) 30 105.00 12.00 104 104 0.89 8.75
Kayaba G30S 30 116.10 16.70 156 156 0.86 6.95
PJ1 Fork Tuner (30wt) 30 116.90 15.00   133 0.87 7.79
Spectro, Golden American 4 (20w50)   155.00 19.60 146 145 0.87 7.91
Credit to the source:  https://bmwmotorcycletech.info/viscosity.htm



Rev:
08/08/2012.  New article.  Discussion section transferred from 54-10A, expanded/revised.  Possible errors.
02/24/2016:  Increase font sizes appropriately.  Move table to left.  Narrow article, left justified. Clean up.  Update meta-codes.
06/24/2016:  Update metacodes, H.L., justify left, layout, colors, fonts, scripts, etc.
01/26/2017:  Clean up.
03/29/2017:  Fix typo (extra 'the') in number 6.
11/11/2017:  Add comments on new 0W16 and similar oils. Clarify some minor details.  Reduce fonts, colors, and html changes.
04/05/2018:  Add chart, re-arrange, fix html;  and add comments.
05/05/2018:  Expand 3, slightly, to include that the measurement is front tire not touching ground, motorcycle on center-stand.
07/15/2020:  Improve clarity of explanations.
08/18/2020:  Again, improve clarity of explanations.
08/19/2020:  Add note.  Remove all Torco chart entries.  Add a link and note.

Copyright 2020, R. Fleischer

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Last check/edit: Monday, December 07, 2020