I have a Troy built pressure washer with a Honda gc 160 5hp engine. I use 5w30 (could be the problem) and the engine oil turns black in less than an hour. I used 5w30 synthetic because my car, lawn mower and generator use the same. I don't remember this happening before. Outdoor temp when this occurred was about 90 degrees and washer was in the sun. Any thoughts? Washer is 12 years old. Other than black oil runs excellent. No black smoke from exhaust and efficient in fuel. P