everyone calls me crazy but i would not , I REPEAT would not change from regular to synthetic on an engine with any miles on it. I had a car that was high miles didnt leak or use oil, went to walmart the little girl ask if i wanted the full synthetic treatment , i was in a rush and said yes, they changed my oil poured 5 quarts in my car , i started it to pull it out and blew the engine . I think the oil washed all the sludge and deposits off the bearing and it spun a bearing right there and then. Just too odd that my engine blew with 100000 miles , i drive 150 miles a day or more all interstate. the exact same engine with regular oil lasted over 300000 miles and was still going strong when i sold it. My new suburban has the 8.00 a quart mobile 1 syn. oil and i use it due to it always being in it but i dont like or use it in anything else...