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Hi, I am unsure of what gear oil to put in my 1970 t100s. Mainly do I need a gl-4 oil, will a gl-5 oil harm it? Also as far as fork oil goes, do I need to get an actual fork oil or will engine oil work, and if it will does it matter if it is single grade or multigrade?
 

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The following are taken from an original factory manual of the period and are correct for the T100S of 1970.

Gearbox SAE 50

Forks
SAE 30 Temp Range (ambient) 60 - 90f
SAE 50 Temp Range 90f+

Both are single grade motor oils.
The fork oil is much thicker than a modern fork oil.
Later the factory started specifying EP90 Gear Oil which is a special "Extreme Pressure" gear oil. This is what I use.
 

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In my 70 T100, I use 80W/90 gear oil (Lucas brand, usually) in my gearbox. GrandPaulZ says 10W/30 in the forks ... I've used 30W fork oil and I've used 30W (straight weight) motor oil. I may try the 10W/30 soon ...
 
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