Engine oil in the gearbox…?

[EuroIron]

they will work good and last a long time on just about any modern lubricant and that is certainly within reason when you consider the superiority of most any modern engine/trans/drivetrain lubricant compared what we would today regard as poorly refined sludge.......... aka engine oil of 60 years ago

there are BMW factory service bulletins suggesting trying any of the following for various manual shift transmissions smitten with various problems

1) engine oil
2)GL4
3)automatic transmission fluid
4)synthetic automatic transmission fluids (I think Redline has been specifically recommended)
5)even hypoid although many who are regarded as experts claim that anything besides GL4 will wreck the very same bushings found inside of many other mechanical devices that commonly use all of the above lubricants and even with the presence of blow by gases, gasoline to dilute things, coolant, sludge deposits........ and a transmission is a surgically clean environment by comparison

a very similar service bulletin exists for the Getrag and ZF's found in modern flagship M6 BMW's as well........ and truly....... not that much has changed in manual power transmission devices in the last 50 years except that M6 BMW transmission costs more than many vintage BMW bikes are worth...... well maybe not but close anyhow

my preference is automatic transmission fluid for multiple reasons and the fact that it is a different color is the least significant of them

it does work very well and will more than adequately lubricate anything found inside that very simple gearbox

viscosity be damned, ATF truly does not have to be compressed to 200+ psi to be an effective lubricant

it's used in very heavily loaded transfer cases (found behind the transmissions of 4WD's) where it keeps everything working good with no pump to pressurize it at all

and in several other similar applications including some of BMW"s own manual shift transmissions

I know this because I am BMW factory trained and not just on motorcycles

and I've also worked on a very wide range of vehicles

so just take your pick but I'd opt against the 80-90-140 hypoid types since those gear boxes really do not need lube that heavy, not even at the equator

[Darryl.Richman]

Historically, up until the mid-60s, BMW recommended engine oil for all lubricating purposes. (They recommended it for their telescopic forks in this era as well.) There's a service bulletin, which mentions particular frame numbers for the R50/2, R60/2 and R69S, after which hypoid oil is recommended. This is supposed to coincide with the introduction of seals that could stand up to the sulfur compounds in hypoid gear lube.

I am not a factory trained anything, and for that reason, I tend to stick close to the original recommendations, trusting in the fact that modern lubricants are greatly superior to what was available in the time. The seals in my bikes have been replaced and so they are all safe for use with hypoid gear lube, so that's what I use.

One issue that often is brought up is the different "weight" numbers between recommended engine oil (30 or 20W50 or whatever you like) and the 90 weight gear lube. I understand that engine weight numbers are determined at operating temperatures, while gear lube is not, and so 50 weight engine oil has a similar viscosity to 90 weight gear lube.

[Allan.Atherton]

That's according to the manual. I found inside mine regular 90 grade transmission oil.... Should I change it? The plate on your motor says it was rebuilt in 1956, when it is also likely the transmission was rebuilt. You should normally be using the motor oil recommended in the manual, unless you know that the transmission was been rebuilt again after 1967 when BMW changed to 90W hypoid (gear) oil and new-type seals compatible with such oil (Bulletin 254 of June 21, 1967).

On the one hand, using hypoid oil with motor oil seals will cause them to leak. On the other hand, if your transmission has 1956 seals, they would probably be leaky by now even with motor oil, so the seals are probably newer.

If your transmissions seals are leak-free now with that gear oil, it is likely that the seals are the post-1967 modern type and you don't have to change to motor oil.

Look under the transmission on the shelf that is on top of the sump. If the input seal is leaking, oil will appear there. If the shelf remains dry, the seal is good.

[Micha]

Thanks guys.
By the way, the engine with that plate is a spare one and not that fitted on my machine. I mean the one with the France workshop plate. I will check those seals.
Thanks again, Micha.

[VBMWMO]

That's according to the manual. I found inside mine regular 90 grade transmission oil. The bike rins OK - Should I change it?