I have a Craftsman YS 4500 yard tractor with about 150 hours on it. I have been changing the oil with the recommended SAE 30 oil, but it is not real easy to find. The book recommends the SAE 30 for above 40 degrees and 5w-30 for below 40. My questions is can I run a 5w-30 (possibly synthetic) in it year around without hurting it? I change the oil about 1 time a year currently and I use a purolater classic "car" filter if it matters and the engine is a B&S Intek Gold 20HP.