Right gear oil

[nudedaytona]

Can anyone recommend what gear oil to use? The bike is a 1996 aprilia rs250 (two-stroke engine) with a wet clutch. The manual says sae 75w90, but the bikeshop reckoned it was too thick and suggested 10w40. I've also heard that I shouldn't use fully synthetic as the additives can cause clutch slippage. Any suggestions?

[nudedaytona]

Just rang mt eden motorcycles. Apparantly oil is rated differently for the crankcase and the gearbox. So an oil which is 10/40 in the crankcase is 75-90 in the gearbox. Not sure exactly how it works, anyway it sounds right so I'll give the silkolene 10-40 semisynthetic a go, which is apparently the equivalent of 75w80 in the gearbox, close enough. It meets API GL4 specs, so that's all good as well.

[Ocean1]

Dude, the viscosity rating id the same wherever you use it. I think there's a misunderstanding there somewhere. Don't know what you should be using in the gearbox but you need to get a specific, correct answer mate.

[Crisis management]

Just rang mt eden motorcycles. Apparantly oil is rated differently for the crankcase and the gearbox. So an oil which is 10/40 in the crankcase is 75-90 in the gearbox. Not sure exactly how it works, anyway it sounds right so I'll give the silkolene 10-40 semisynthetic a go, which is apparently the equivalent of 75w80 in the gearbox, close enough. It meets API GL4 specs, so that's all good as well.

This is utter bullshit.

As Ocean 1 said, the SAE viscosity rating rates the oils ability to move quickly ie. thick or thin oil, and this is a constant for that oil, it doesn't change dependant on its usage.
If the spec says 75-90 in the gearbox you better use that, 10-40 will not cut it in there.
As far as using semi or full synthetics....for how many years now have we had wet clutches and those oils being happily used together? I think you can probably safely use either in the gearbox!

EDIT: Looks like I'm wrong (again)....thanks Motu for the clarification.

[Motu]

No,Mt Eden are correct,gear oils are rated differently.....EP90 has the same viscosity as SAE 50 engine oil.So 75/80 is the same as a 40 weight...10/40 is pretty close.A 75/80 GL4 is excellent for a gearbox,and is safe for clutches.Most bikes manufacturers recommend 10/30 or 10/40 engine oil,but make sure it has no friction modifiers....10/30 may do,but not likely with 10/40.Better yet go for a HDEO (diesel oil) and find one rated for Allison (most are),that means they are good for gears and clutch packs.

[vagrant]

Check this page for a full spec sheet for the RS250,

http://www.vsource.org/VFR-RVF_files/Brs250-mi.htm

Under transmission it says to use "IP FC, SAE 75W-90, or any high-quality oil meeting the API GL-4 specification"

Shell have a handy fluid finder tool on their site, here's what they say for your bike,

http://www.shell.com/home/page/nz-en/shell_for_businesses/lube_match/app_lubematch_1128.html

That should sort you.