Oil Changes . . . How often?

I can`t tell you how many cars I`ve changed oil on where the customer uses the manuf. guideline or the cars oil life monitor to tell them when to change the oil. the oil comes out as black as a diesels engine oil. you look down inside the valve cover and all you see is gunk and the formation of sludge. I change my Mobil 1 in my Excursion around 4500 miles and my minivan with regular oil at 3000-3500 miles. the inside of all my engines are clean! even my old 190k mile minivan was clean inside!

I change the oil by what I know and see! 7500 miles on a gas engine oil change is just asking for trouble.........usually after the warranty runs out!