Hey guys: When I first installed the Castrol 0W-30 as a replacement for my Ford 5W-20 I immediately noticed a quieter running engine. I also replaced the K&N filter with the original Ford Motocraft air filter. The GC poured noticeable thicker than the Ford 0W-20. Here's a 1 month conclusion: The car feels like its lost some hp. By the way, my car is a 2000 Ford Mustang GT so each HP counts. I'm making 60 miles per tank less with this new setup. This is with 5 different gas fillups. Is it the oil or the air filter? Cause I'm a little pissed about spending so much extra for Castrol 0W-30 and a K&N oil filter. Any advise or comments, Thanx.