Gearbox oil?

[NeilS]

What are y'all using for gearbox oil? The DVD service manual says SAE 90 GL-5, which is just about impossible to find these days. Jim vonBaden's DVD shows him putting in Castrol Syntec 75W-90 (although it's into a GS). And I seem to recall Don advocating 75W-140 somewhere on here that I can't find.

Rooting around the garage, I found a bottle of Redline synthetic 75W-90 GL-5. That should work. Right?

[ka5ysy]

I use Amsoil 75-140 GL5 synthetic. You can order it online from Amsoil. Works great and reasonably priced.

[sandycruz]

Here is a link to a great "how to change the gearbox oil in an RT". Same should work for our R12R.
Thanks to Don!

http://www.bmwmoa.org/forum/showthread.php?t=31056

[deilenberger]

Here is a link to a great "how to change the gearbox oil in an RT". Same should work for our R12R.
Thanks to Don!

http://www.bmwmoa.org/forum/showthread.php?t=31056

That's thanks to Luis who did the DIY's - I just posted them for him..

[NeilS]

Don,

I can't seem to get the links to work from the MOA site. No problem, I can figure oil the gearbox oil change. But perhaps you can explain your (or is it someone else's?) rationale for 75W-140 oil in the gearbox.

TIA,

[deilenberger]

Don,

I can't seem to get the links to work from the MOA site. No problem, I can figure oil the gearbox oil change. But perhaps you can explain your (or is it someone else's?) rationale for 75W-140 oil in the gearbox.

TIA,

Neil,

Are you a member of the MOA? Some portions of the website are restricted to members only..

I haven't done 75W-140 in my gearbox - but I have considered doing it. I'm due in about 1,500 miles for my 24,000 mile service.. and that will be when the transmission oil gets changed.

Past experience with BMW (Getrag) gearboxes has been - in mid summer, when it's HOT, and the traffic gets bad, and I've been sitting in traffic for a bit, the gearboxes shift smoother with the more heat resistant oil in the box - less bang-klunk. When you have a gearbox that has a hot catalytic converter sitting right under it - the effect is probably more noticeable.

The thing to remember about multiviscosity oils is - they don't actually get thicker as they get hotter - they simply don't get AS thin. The rating is 75W (Winter) - 140. What that means is the oil acts like a 140 oil would WHEN it gets HOT. The actual viscosity HOT is likely to be much less than the 75W viscosity in winter. The common misconception is that the oil actually gets thicker as it gets hotter. It doesn't - and that's why it's recommended to drain oil hot - that's when it's thinnest, no matter what multi-vis it is.

So - my considered opinion (and that's all it is..) - is - it would do no harm, and might make the transmission shift smoother when hot to use the wider viscosity oil in it. BMW does sell a 75W-140 oil, specifically for transmission use - right in your friendly dealers parts department. Seems odd that they do since none of the bikes specify the use of it, but there it is..

[NeilS]

Don,

Interesting thoughts. So you're saying, in effect, that BMW specifies a lubricant to have a certain viscosity range over the normal temperature range of the gearbox. That range includes, at the low-temperature end, the viscosity of 75W oil, and, at the high-temperature end, the viscosity of 90 weight oil. So, by going to 75W-140, your oil is further away from the low-viscosity limit at high temperatures that it would be if you used 75W-90, and it would therefore provide better protection to your gearbox. That makes sense.

The counter-argument might be that the 75W-90 oil might be "stronger" than the 75W-140, and might retain its viscosity characteristics longer over time. Before synthetics, the conventional wisdom was the smaller the "spread" between the low-temperature and high-temperature ratings of an oil, the more stable the oil would be over time and temperature. That was because the spread was achieved by the use of "viscosity-index improver" additives, and not just the fundamental characteristics of the base oils. The VI improvers degrade over time, so an oil with a lot of VI improver would degrade faster and more severely than an oil will less VI improver. I don't know to what extent that argument is modified by the use of synthetic base stocks.

Thanks for the explanation.

[Bill Stevenson]

Neil,

Your understanding parallels mine. Essentially the viscosity index improver that was used when they first came out some 40 years ago or so was a Styrene. The way it was explained to me was that when the oil was cold, the viscosity index improver was sort of like a microscopic tightly wound inner part of a golf ball. As the temperature rose, the polymer expanded and looked sort of like a microscopic plate of spaghetti. Hence with polymer expansion as temperature increased, the polymer additive caused the lubricant to flow more slowly through a viscosity cup. This was so as compared to the same base stock oil without the additive package at the same elevated temperature.

Originally the problem was that the Styrene was quite susceptible to shear and the strands quickly broke up. Some may recall a time when GM would not honor warranties on engine failures (mostly sticking piston rings and loss of compression) if 10W40 oil was used. The worst situation for the early polymers was in a gearbox. Over time the Styrenes were replaced with Olefin based polymers which were much more resistant to shear breakdown. This allowed the use of more of the additive and broader viscosity ranges without the attendant problems and the controversy faded away. As a rough rule of thumb the amount of viscosity index improver doubles for 10W40 or 20W50 as compared to 10W30 and 20W40 respectively. It should also be mentioned that viscosity index polymer packages are used for both fossil base stocks and synthetics.

Just speculating based on this explanation, it seems to me that in a gearbox with the greater shear forces involved, the use of a limited viscosity range product would result in improved life of the lubricant. Conversely an extended viscosity range gearbox lubricant would breakdown more quickly and should be changed more frequently all else being equal. My bias is to follow the manufacturer's recommendations.

Regards,

Bill

[deilenberger]

Don,

Interesting thoughts. So you're saying, in effect, that BMW specifies a lubricant to have a certain viscosity range over the normal temperature range of the gearbox. That range includes, at the low-temperature end, the viscosity of 75W oil, and, at the high-temperature end, the viscosity of 90 weight oil.

Almost.. I'm not always sure I'm clear. My understanding of what it means is:

Cold/low temps - the oil has the viscosity of 75W oil at that temperature - ie - it's a bit thinner than 90W single-viscosity oil would be at that temperture. The single viscosity oil will get thicker (more viscous) faster as the temps drop. Both oils will still be somewhat thicker than they would be at room temperature.

Ditto on Hot temps - the oil will have the viscosity of 140 weight oil - of the 140 weight oil at that temperature ie - it doesn't get thin (lower viscosity) as quickly with temperature increases. It's still thinner (lower viscosity) than it is at room temp, but less thin than the 75W-90 would have been.

So, by going to 75W-140, your oil is further away from the low-viscosity limit at high temperatures that it would be if you used 75W-90, and it would therefore provide better protection to your gearbox. That makes sense.

That's pretty much the idea... I found quite a noticeable improvement in K bike transmissions, and on my R1150RS transmission - when the temps got hot and when I was stuck in NJ traffic.

The counter-argument might be that the 75W-90 oil might be "stronger" than the 75W-140, and might retain its viscosity characteristics longer over time. Before synthetics, the conventional wisdom was the smaller the "spread" between the low-temperature and high-temperature ratings of an oil, the more stable the oil would be over time and temperature. That was because the spread was achieved by the use of "viscosity-index improver" additives, and not just the fundamental characteristics of the base oils. The VI improvers degrade over time, so an oil with a lot of VI improver would degrade faster and more severely than an oil will less VI improver. I don't know to what extent that argument is modified by the use of synthetic base stocks.

Thanks for the explanation.

Absolutely correct on the breakdown of VI additives - and a more important factor with non-synthetics, since the viscosity of the base oil tends to break down faster on non-synthetics. FWIW: I know of no single-weight synthetic transmission gear oils. Might be some available, but everything I've seen has been multiviscosity.

[NeilS]

You said it right the first time, Don. My response must not have been clear

And I agree that you can't find straight 90 very easily any more. So you have to wonder why BMW calls for 90 weight in the service DVD. Is it more available in Europe?

[motoracer8]

On the subject of oil, I say never argue politics, religon, or the brand of a mans motor oil, because that falls back under religon. Seriously let me say alittle about gear oil. Straight weight gear lubes are hard to find now at a retail level, they are still made, but mainly for industrial applacations. The multi vis gear lubes for general automotive use are much better than the old straight weights any way. When resurching lube oil specs. motor oil, or gear lube, look for a high viscosity index, this is not the sae numbers you see on the container, ie. 75w/90. The higher the VI number the more stable the product is, Group 4 &5 base stocks have generaly high VI numbers and so do most synthetics. What this means is not nearly as much polymer is needed to keep these products in grade. There is still degeneration over time but it is much slower than just a few years ago. If you take a look at hydraulic oils, or even the beter fork oils used in motorcycles some of them have VI's over 300, thining from heat is minimal. Motor oil, and gear lube base stocks have VI's in the 130 to 175 range, the higher the number the beter. Years ago polymers were not of quality they are now so the products were not streched as far, hense sae 10/30 or 20/40 or 80/90. I seriously doubt you would see any ware difference between using 75/90 vs 75/140. Just for alittle info 75/90 gear oil is just about like 20/50 motor oil, thats what happens when you get engineers together. SAE 20W motor oil and SAE 75W gear oil have just about the same range as dose SAE 50 motor oil and SAE 90 gear oil. Gear lube numbers go all the way to SAE 250, but these are special industrial applications. I could go on here for pages and I am shure it would bore every body, but it is entertaining to read a lot of the nonsense about verious lubricants on auto and motorcycle web sites. I worked for a oil company that made products for the performance auto and motorcycle industry, we private labled some fairly famous brands over the years, some still here many long gone, some very good, and some not so good.

Ken G.