My 98 just hit 150,000 miles, and I'm thinking about changing over to using a high-mileage synthetic instead of the standard oil I've been using. I dont think it's necessary because I know how tough these engines are, but considering the fact that I do need a couple more years out of it and that I've been adding more performance parts to the engine (intake, exaust, new injectors, ect...) it seems like a good idea. However, I've been told the XJ isn't going to like it if I switch over to a synthetic, doesn't make sense to me but I wanted to be sure. So if any of you have switched to high-mileage synthetics or know anything about this I would appreciate your help.