737, follow Test's advice on the rpm range - a 12x6 APC is a good place to start.

Also, check the exhaust system and ensure that the locknuts on the exhaust port and muffler are nice and tight. Dark, metallic oil is often a sign of loose exhaust componentry. A four-stroke's exhaust output "spooge" with full synthetic oil should be pretty clear, perhaps with a tinge of brown when run hard.