I've got an 18hp Briggs opposed twin vertical shaft on a 1990 Craftsman garden tractor. The engine owners manual shows that SAE 30, 10W-30, or 10W-40 can be used for summer operation. It has about 500hrs currently. What's everybody's thought about using 15W-40 or synthetic 5/10W-30 in it? Currently I've been running conventional 10W-30 and always stays at the full mark on the dipstick between changes. Thanks