As long as I have been doing repairs and going to mfg tech schools, they have always harped on the general rule of thumb, if the average temp during operation is 50 deg f or above use 30W, below that 10-30, 10-40 for OHV with oil pump and filter,only 3 types of oil I ever stocked or sold in the shop, I have always gone by this and not ever had an engine go bad on me. I have a 87 snapper push with a Tech TVS 105 on it and it doesn't use any oil, I change the oil every spring (just a trim mower for the last 10 yrs). My brother is a synthetic oil nut abd complains all the time that his engines burn oil and has to add oil every 2 or 3 times he uses them.