I'm using 5w 40 Quaker State. Its really helped quiet down the sometimes loud idle noise - it sounded like Thor was hammering in there sometimes, and its a lot less frequent since I switched about 1000 miles ago to syn. BTW, my pal who has a Volvo Turbo which has used syn since the beginning has over 275,000 on it and never done work on the guts of the motor. An odd water pump or other external fix, but never the internals. Pretty good stuff.