I changed the oil for the first time on this car (a 1990 300zx non-turbo)last weekend, swapping out from what the previous owner told me was m1 5w30 for Castrol GTX 10w30. The old oil had 5k of short trips on it, but wasn't too nasty. Over 4 quarts came out between the filter and drain. I put in 4.3 quarts of the GTX, which the cars manual asks for 4 and 3/8 quarts. A week later, I notice that the MPG has dropped at least 3 mpg, with the guage showing more than half empty after only 115 miles or so. Before the OC I was getting close to 280 miles out of 3/4 of a tank. What gives? There may be some sludge in there which is causing this loss, though I don't believe the oil level is too high( dipstick has always been unreliable). Oil pressure is higher than before, over 60 psi idle at cold startup. Any suggestions or comments would be apprieciated, thanks.