Hi,I have a 1985 ty250 mono and I am changing the gearbox oil,question is the manual states 10/40,is this the best to use or is light gear oil the way to go?.thanks

Personally I have always used ATF in all my trials bikes.

10W - 40 has a viscosity of about 11 to 15 when hot, ATF at the same temperature only about 5 or 6 so not really what Yamaha intended. Yamalube 10W - 40 or any good quality 10W - 40 semi synthetic is best.

 

75W light gear oil will lube the gearbox but may not be compatible with the clutch lining material. The EP additives in some gear oils attack the clutch friction material.

Make shure it's a 10/40 oil meant for motorcycles, automotive oils have different additives that are not intended for cycle use. May mess with clutch plates

Do as Yamaha say and all will be well.

 With my ty`s I always used Bel Ray gear oil. That was long before I started using ATF. No problems. Can`t go wrong with yamalube.

Thanks guys,will get some oil sorted.

I realize this is an old post, but I had the same question, and an old post recommended Silkolene Light Gear oil. I tried some and it did not work well with the clutch. The bike shifts well, but the clutch drags, as in pull in the clutch, and the bike continues on a bit. Plus, when cold, you better have it pointed in a safe direction when you put it in gear, as it's going wherever it's pointing. Breaks loose in a few seconds, but a little scary when it happens. 

I've been using Spectro Clutch Saver in my two ty175's.  There's a moderate amount of drag when the engine is cold in one of them. Hardly any drag with the other one. I think that I used a surface plate and sand paper to dress the plates in the better behaving at some point in the past to insure they were flat.

Yamaha Enduros from the same era as the ty175 had special rubber bands to keep the plates from sticking together.

Read the Beta clutch fix by dan William ( in the Beta section ) it does apply and it's worth the effort.

 

Guy