Just found this site and have to say A+… I have been lurking for a couple of days and had a question… I have three vehicles that are nearly identical, and I am considering an extended oil change interval. What is the community’s suggestion? DETAILS 1995 GMC Suburban 4x4, 5.7L engine (225 hp), auto transmission, 120,000 miles,15MPG, throttle body injection, weighs 8,000 lbs, turns 1850 rpm at 60 mph, uses 1.5 qts of oil in a 3,000 mile oil change. Average trip is 20 to 40 miles twice a day. 1997 GMC extended cab truck 2 wheel drive, 5.7L engine (250 hp), SFI, auto transmission, 16 MPG, 148,000 miles, weighs 4,400 lbs, turns 1850 rpm at 60 mph, uses no oil. Average trip is 5 to 40 miles twice a day. 1998 Chevy extended cab truck 4x4, 5.7L engine (250 hp), SFI, 5 speed transmission, 18 MPG, 142,000 miles, weighs 4,400 lbs, turns 1500 rpm at 60 mph, uses 1 qt in a 3,000 mile oil change. Average trip is 20 to 40 miles twice a day. I use Havoline 10W-40 in the summer (average low = 65F, average high = 90F) and Havoline 10W-30 in the winter (average low = 20F, average high = 35F) and along with AC Delco oil filters. Is it safe to go 4,000 miles on the oil in each vehicle? What about the suburban? It is twice as heavy as the other vehicles, so the engine has to work harder?