Snowbum's BMW Motorcycle Repair & Information Website

For proper engine operation, lubrication, protection, etc., engine oil should be in an approved temperature range after the engine has warmed up; not too cold, not too hot.  The oil also needs proper viscosity over its entire expected temperature range.  Usually, a multi-grade oil such as 20W50 is used in our Airheads.  Quality 20W50 motorcycle oil is far better than single grade oils. A thinner oil might be wise if temperatures are near, at, or below freezing; perhaps a 10W40.   10W50 and 15W50 oils are available.   Further information is here: https://bmwmotorcycletech.info/coldstarting.htm  as well as my various oil articles, including https://bmwmotorcycletech.info/viscosity.htm and https://bmwmotorcycletech.info/oilessay.htm.  Please refer to these various articles, there are at least 5.  See section 50 and 51 items on this website, available on the Technical Articles List page.

Motor oil in your BMW Airhead engine works best when it is ~180 to ~235°F in the engine oil pan.   The oil is considerably hotter at the valve guides & other areas of the cylinder heads where a small amount of oil circulates slowly at very hot places.  There is some deterioration of the oil, from evaporation (particularly of oil additives absolutely needed and so part of the oil you purchase).  Certain types of combustion products also age the oil, and even leave deposits, typically carbon. Overall, the deterioration is slow, mostly because only small amounts of the oil are exposed to high temperature for any period of time. Because oil (and especially its additives) deteriorate with use, oil is changed regularly ....depending on miles, time, & conditions of use.   You need a high enough oil operating temperature to ensure good oil flow & good lubrication, but if the temperature of the oil is considerably too high, the oil deteriorates rather fast, and even typically gets 'burned' faster. Bulk oil temperature needs to be high enough to evaporate the vast majority of moisture that has accumulated.  That accumulation happens at every start-up and cool-down. Some oil additives help with moisture problems, but there are limits to the effectiveness.  Petroleum oils increasingly deteriorate above 235°F.   The deterioration rate is on a relatively steep curve, & is very bad by perhaps 300°F.  For this, and a few other reasons, many folks use part-synthetic or full synthetic oils. Because the required additives evaporate and deteriorate over time and temperature, oil change intervals can not be overly extended by using synthetic oils.

Airhead production began in late 1969 with the /5 series.  Oil coolers were not fitted, and were generally not needed, as there was plenty of fins, etc., to enable adequate cooling for normal riding situations.   As time went on engine sizes grew; power output increased & fairings were installed on some models.  More engine heat was developed & trapped from these things, particularly at higher speeds.  While kits to install oil coolers were available early-on for those having special needs, such as for racing, it was not until 1978 that BMW began incorporating them into production models, with the R100S, R100RS & R100RT.    Many have converted other, especially earlier models, to have oil coolers.  The faired models, especially the RT, do not get as much over-all cooling as the naked models, so coolers are standard for the RT (and RS).    BUT...oil coolers are not always necessary.  Contrary to some opinions, the engine oil does not usually get extremely/excessively hot in most city driving ....because low power is usually being used, as opposed to high power for high speeds.  In severe conditions, such as extended stop and go, quite hot weather, and/or high power output, the oil (and the motorcycle engine) can get hot enough to benefit from an oil cooler. Thus, BMW incorporated an oil cooler on some models especially so-affected, such as the faired models, and in some sport performance models, like the R100S.  The benefits from using an oil cooler when appropriate is not only good for lubrication purposes and extending the oil additives life, but it helps lower excessive engine temperatures, decreasing wear.  Although not often thought of in this way, the Airhead engine is both air-cooled and partially oil-cooled.  Even though the effect of the BMW oil cooler on oil temperature is modest, the over-all effects on the oil and wear, etc., are fairly high.   Note that system oil that is too hot by even 10 or 15 °F, is helped considerably by a cooler that lowers oil temperature that amount.   Over-all, the oil needs to have many proper characteristics, but the basics are proper viscosity for atmospheric temperatures expected, a viscosity index that ensures reasonable viscosity over the expected temperature range, additives that do many things.....and this list could go on and on.   Lubrication theory, presented in my oil essay article, is ...or can be...very complicated.     You, as the owner/operator, need only to use a good oil, rated for your motorcycle, and change it on a proper schedule.    Common automotive oils do NOT meet the needs of your Airhead motorcycle!  This is especially due to the type of cam and cam follower, used on your Airhead.   There are many cars that were made decades ago that have the same oil requirements as your Airhead.

Description of the BMW cooler as installed by the factory: