Fork Oil

[JohnT]

Sorry to bring this perianal up again but what's the current thinking on fork oil?
I always got by with automatic transmission oil.
Bike in question is R50/5.

[Kurt in S.A.]

More than you want to know here on Snowbum's page:

https://bmwmotorcycletech.info/front-fo ... mounts.htm

I would think this is a simple thing to get done. If you're a road racer or push the bike to its limits you might need to do some serious research and testing. But for the most part, using a solid product like a Bel-Ray in a 5w, 7.5w, or 10w would work quite well. YMMV.

Kurt

[JohnT]

Thanks Kurt. Amazing amount of detail!

[Kurt in S.A.]

You can say that again! Hopefully it's detail that you can use.

Kurt

[Rob Frankham]

The 'W' rating of fork oil is not actually very helpful, except in general terms. It describes the viscosity of a given oil at 0 degrees c and has little bearing on the performance at any other temperature. BMW recommended specific oils for forks varying between 2.5W and 10W with many that didn't quote a 'W' rating at all. Most of these oils are no longer made. In general, most users find an oil somewhere between 5W and 10W is best but it is a good idea to do some experimentation to find what suits you. Once you have it, stick to viscosity and brand for consistency.

FWI, I find a 7.5W produced sold at the local auto spares store (Halfords in the UK) works as well as any and that's what I stick to.

Rob

[barryh]

The 'W' rating of fork oil is not actually very helpful, except in general terms. It describes the viscosity of a given oil at 0 degrees c and has little bearing on the performance at any other temperature. BMW recommended specific oils for forks varying between 2.5W and 10W with many that didn't quote a 'W' rating at all. Most of these oils are no longer made. In general, most users find an oil somewhere between 5W and 10W is best but it is a good idea to do some experimentation to find what suits you. Once you have it, stick to viscosity and brand for consistency.

FWI, I find a 7.5W produced sold at the local auto spares store (Halfords in the UK) works as well as any and that's what I stick to.

Rob

Fork oil manufacturers use of the W designation is even more misleading than that.

For normal multigrade oils, W officially stands for winter and it it's a measure of viscosity at very cold temperatures.

Fork oil manufacturers seem to be using W for weight which has no official meaning. What they are doing is using W as an approximate indication of the the viscosity at 100 Deg C which is not helpful either. What I always do to compare forks oils is look up the data sheets to find the viscosity at 40 Deg C which is much closer to real world operating temperature.

Screenshot 2026-01-17 124434.png (18.88 KiB) Viewed 1914 times

[Kurt in S.A.]

Sort of like selling Cheetos without any real cheese!!

Kurt