Started using synthetic oil in a new 98 windstar after 500 miles,changed oil every 5000 till 50000 miles then every 10000 miles,at 125000 when we sold it(kids grown so no need for van)motor was spotless looking in valve cover thru oil fill and ran like a top!!Never used a drop of oil, fast forward just got my first flathead(new rebuild) ran for 500 miles with 30 non detergent with zinc additive. never believed synthetic would cause a leak over petroleum or vise verse. had a small leak at the rear of the pan (typical). just changed oil (10-30 synthetic Kendall) guess what? No leak!! Beats me but I'm happy!! Pete