I used to use Valvoline, plain ol 5/10w30. Kept it in my 175k or so SC400 with the OEM filter when I got it and always changed it at app 3k and it always came out looking just a shade or two darker than when it went in. Then I got my 300ZX and I've used Amsoil 5w30 and Amsoil filter. It's finally about time for the second oil change, but sticking with it.