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