I did a lot of thinking about this when i did my swap and now have a solid wheel. I tried Redline manual tranny fruid. That didnt seem to help my roll over noise any. I tried going to my local big rig place and getting some atf fuild. ( i cant remember what wt i got though). I didnt like the way it shifted with that. My tranny has many many many miles on the syncros so i cant say which is better. In a couple weeks here i will be pulling my tranny apart for a syncro kit and to inspect everything else. (already put a bearing kit in it before i knew what roll over noise was).
So with that thought here in the motorcycle industry we use lots of oils that deal with trannys and clutchs. We can not use a regular car oil because most of the bikes use the same oil for the tranny and engine. So for instance honda makes an oil that is a basic 10w40 and is aproved to be run in a bike that the oil goes through the engine and tranny/clutch. Honda also makes a tranny oil that says 80/85 wt on it for the 2 stroke bottom end. Now i know that oil is not 80/85wt because its like water and has something to do with being rated differnt. Bell ray has a gear saver designed for tranny/clutch as well thats an 80/85.
Would a basic dino 10w40 motorcycle oil work for something like this? or is it too thin?
Your regular modern motor oils are not the best as there are some additives that do not play nice with the synchros long term. You will notice when Mel mentioned motor oil, he specified 30 wt *non detergent* that is a beast that is getting pretty hard to find aside from some small engine specific oils, but it it absent the offending additives that can damage the synchros. You will not find a multigrade motor oil that does not have a detergent package. and gear oil is acceptable as well, but it has to be gear oil that doesn't have EP ( extreme pressure ) additives in it as , you guessed it, they don't play nice either and they can have a habit of helping the tranny jump out of gear
If you went to a big rig place and they sold you some fluid and it didn't shift good, there's a good chance you got some old school yo-yo who sold you a jug of mineral oil if you didn't know for sure what it was you were getting. You can always look up the specs on whatever it is that they have. The manufacturers always post the specs on teh web
When the zf came out synthetic wasn't real popular and it had dino atf in it and that was the spec and regular atf is 20 wt , they later changed their *primary*spec ( I say primary as zf has a whole list of acceptable lubes for their boxes including the zf542 )for diesels towing to be synthetic ATF but if you were to go to a 30 or 50 wt synthetic atf your shifting , especially cold should be way better than what it would have been on the original fill. A 50 falls under the high limits of the highest spec for a gear oil fill, but that is also using dino lubes as a standard and not synthetics, so a 50 wt synthetic atf shifts cold smoother than a dino 20 wt so you don't have to spend the first 15 miles double clutching the thing trying to fight it into gear due to to thick a lube like you would with gear oil ( if you were having to do that, somebody sold you some mineral oil)
Take it from someone who followed that path of thought and action with transmissions before and found that thicker lubricant impaired the function of the synchronizers to a really noticeable degree, making shifting much more tricky and harsh. I am actually glad that the difference was so noticeable, as it made me change it right back to the recommended fluid, whereas if the difference were slight, I might have been accelerating wear without knowing it. We all like to "improve" things but sometimes you have to remember that OEM recommendations often have a basis in a depth of detail and experience that is not apparent to anyone else.
