I checked and the last I bought was straight 90W and when I change the oil out that is what I will most likely use. I have not tried synthetic. The old Vicon mowers I used to use were the same way on oil use. I think all the brands that have an oil bath bar are pretty much the same way. You might see a slightly "oily" look on the end of the bar where the vent is. You should not see any seepage or oil stains on the top of the bar around the rotors.