Here is a deal. My father has 06 Nissan Frontier with 2.5 L engine. It has 91k miles on the clock. He doesn't drive it hard, and mostly highway miles. For couple years his roundtrip to work was 80 miles a day. This truck has some moderate loads sometimes, like furniture, motorcycle,lawn mover, drywall, tiles... So it's not driven very hard. He claims he used syntech oil from start,he got it brand new. His OCIs are 10k miles. I'm doing changes for last couple years. It usually gets whatever syntec oil is on sale. Usually Castrol,M1,Penzoil,Valvoline. Month ago I seafomed it 3-way, before change, put in PU and can of Moly. Is he will be fine with 10k ocis? And I was thinking switching him to HM syntech like M1. Is it time? I'm not sure is there any varnish or other deposits,can't see through long filler hole. Will takse closer look during spark plugs change out.