I change my oil and filter every 50 hours in my 23HP Kawi. I also have a Engineaire air filter on it and check it regularly. Here's my problem/question: After about 5 hours on new 10w30 Valvoline oil it turns a dark color like it was dirty and ready for a change. You can still see through it on the stick, but it is almost black. I thought that maybe not all of the old oil was draining out, so the last time I changed it I just about tipped the machine over so that it would all drain. It made no difference. Anyone else notice this with their Kawis?