I have been using 5w-20 royal purple in my 2004 Mustang
GT (86k miles) with a k&n oil filter and I've been changing it every 3k to 4k miles. My question.. Is this over kill? I usually use Z-Max in my other cars but for this car I've only added in Lucas oil stabilizer. To my knowledge it hasn't burned off or leaked any oil since I've had it. Also I was told that it has only had synthetic oil for the life of the car (or so I hope).