I tend to keep a simple process simple. Whatever the manual recommends for your gennie is what you should use. Ours calls for 15w-40(Good quality Dino oil such as Shell Rotella) and I have always used that with 0 problems. The only exception for me would be if I were to be in extremely cold weather then most manuals will suggest a lighter weight oil. To me, the use of anything else such as synthetic oil seems to be a waste of money as you should be changing the oil yearly anyway. The purchase of the oil filter wrench for removal would be a smart purchase.