Originally Posted By: Sealbilly
Thanks gentlemen. I read my owners manual for the Toro I just bought and it shows 10w30 but 30 wt for higher temps. Was just curious to why it doesn't call for 10w40 or 15w40.
Air cooled OPE engines use the motor oil as part of their cooling systems and they run VERY hot under high loads in warm weather, hot enough to break down the Vii additives in multi-weight motor oils. In 10w30 for example, Vii is used to keep a lower 10 weight oil thicker when it gets hot so it performs like a 30 weight. When the Vii breaks down completely you are left with 10 weight oil and sludge. As a general rule, the wider the gap between the numbers, the higher the amount of Vii is in the oil. Also, Vii additive has NO lubricating properties of it's own. Modern Vii additives are much better than they used to be, and most synthetic motor oils use less of it, but the Vii will still break down when subjected to very high heat. HD SAE30 motor oil does not have any Vii additive in it so it performs better, protects better, and lasts longer in very high heat applications.