I run Pennzoil 5w30. I have had excellent luck. 238k on a 92 4.6. 176k on a 90 3.8 300+ on a 80 300 and lots of other Fords with lots of mileage. I change 1 time a month and run about 1200 miles a week. On the cars that I don't run that hard I change every 3000 miles. I think the frequency, quality of filter and changing while it is hot is the key. I have thought about going synthetic as well, but can't find the value in it yet.