I haven't pulled the valve cover from my4.0 litre engine, but I have periodically shone a light down the oil filler hole and taken a peek inside. I don't see much sign of anything nasty like sludge, or even a lot of varnish. I've been using Castrol GTX for 86K miles, but after reading all the previous discussions, I'm thinking maybe I'll change. But to what? I'm not sure I want to go with Mobil1 or another synthetic. And I know everyone has their personal opinions and favorite oils, but realistically, what dino oil is a better choice?