I never use conventional as even regular changes leave behind goo. I got a car from my grandparents my grandpa always changed her around 3000 miles as far as I know. When I took it over of course I used synthetic and you should see some of the crap cleaned out. Looked like blood clots in the filter. I am almost to my third pile change and things have cleaned up some but still a process. Once that sludgey junk is in there it is hard to clear out. It is a 3.1L with around 112,000 and hopefully with regular changes I can get it nice during a regular interval. Group 3 synthetic even is far superior to conventional.