A friend of mine has been using Mobil 1 5w30 (full syn) for about 30,000 miles on his car. He never takes Synthetic past 3,000 miles just because it didn't "feel" right. Now, he switched back to regular oil (Havoline) because it costs a lot less. He has like 35,000 miles on his car. What are the possible negative effects on his car? Is this a big no no?