When I had my 1972 Ossa 250 I always mixed one quart to 5 gallons. This made it easy to mix since I had a 5 gallon container. This puts the ratio at 20:1. Never had any problems with it. For the transmission lube I want to say I used 30 weight motor oil. Don\´t put any additives since the clutch uses the same oil and the additives will mess up the wet clutch 
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