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Front Fork Oil discussion; filling amounts; hints.
This article applies to Pre-Airheads, Airheads, Classic K bikes 
section 54-10B
Copyright 2017, R. Fleischer

Discussion of Oil types and characteristics.
See also:

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 fork oil.   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 vastly 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-5606E.     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 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!).

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:


Except for some GS models, all the BMW's (that this article applies to) need a rather thin oil, approximately SAE 4 (that military oil, as above), so do not willy-nilly change the viscosity grade to 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).


A fairly high viscosity index is generally a good thing for fork oils.  The viscosity index of an oil is simply how fast or how slow, with temperature rise and fall, does the oil thickness change.  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):  Once the VI is high enough, you won't notice any difference in performance by going higher, unless doing VERY aggressive riding in VERY bumpy & constantly bumpy extreme conditions, such as RACING, and even then, only noticeable between early (cold) forks and forks warmed by lots of use. BMW Airheads forks from 1981 are especially sensitive to fork oil foaming.  I suggest you use a quality FORK or SUSPENSION fluid (oil).

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 fork oils if they got thinner as they got colder.  Multigrade oils don't really do this like you might think, they simply change less in viscosity over a temperature change.  Quite an improvement in fork action might be possible with oils that worked in reverse to their normal characteristics.  Still, some some respects...will be possible by using multigrade engine oils, assuming they are compatible in other respects, with fork innards.   Just since early-2017, several new oils have come onto the market due to changes in car engine manufacturer's specifications, in the ever-constant attempt to reduce friction.  One of those oils is rated at 0W16.  We already have had very widespread use of 10W30 and 5W30.  I think that some 0W16 or other quite low viscosity engine oils are going to end up in front forks, and possibly when rebuilding rear forks.  I have not yet done testing, but am intrigued by the idea of getting a nice smooth semi-soft ride, that still performs excellently over big and tiny road irregularities.  Stiction must be considered.  Meanwhile, the best we can do is to get a fork or shock absorber oil that has a reasonably high viscosity index.

Comments and chart of oil filling amounts:

1.  In instances where I know the information, I have listed oil refilling amounts for both a drain/refill; and, a fork rebuild.  A 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 in the forks before you fill them with the proper oil amount, thus the rebuild oil amount is always less than a drain and refill amount.

2.  BMW has revised oil amounts for some motorcycles, which is in my charts.

3.  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.

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

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

6.  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.

7.  Mind my comments about using a quality oil with no tendency towards foaming.

8.  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 heavy 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; slight assembly oil only, otherwise dry




1968-1969 U.S. models




150 to 170

/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



R45, R65 to 1985, R65LS, R80ST



R80 from 1985, R65 from 1986


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


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



Classic K bikes:    



K75 standard


K75 Showa, as on K75RT, etc



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


K100 (8 valve)


K100 (8 valve) RS, RT, LT, standard


K100 RS (16 valve)



K1100LT, K1100RS

Left 350; Right 400

400, 400


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.

Copyright 2017, R. Fleischer

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Last check/edit: Monday, January 15, 2018