I recently bought a 1992 Stock NSX. The original owner used nothing but mobile one since birth as best I can determine all the way through 94,000 miles. A second owner has put regular oil in up to 100k even (netting 6k miles on reqular oil). Should I switch back to synthetic? Engine does have a slight taping noise but I'm not familiar enough to know if that could be the reason. Cost is not especially a concern but I've heard changing back an forth is not a good idea. Thoughts??