Me, 5-40 Castrol or Mobile 1. I have used Castrol for the past 30 years on most of my cars. Tires, I recommend Falken 912's. Very nice smooth ride and perfect for warm and wet climates...terrible in snow country. I just put them on my 2000 Boxster S in November. Love em. $625 installed from Discount Tires and I have 18" on mine and the backs are larger than the fronts.