Hello. I have a 1986 6.2 diesel Chevy Blazer k5 4x4. It now has 380 000 miles on it not km. It’s hard on oil generally need to add 1 quart of oil every 300 miles. No leaks and no smoke at tail pipe. Engine never open. Still original. Yes lots of miles but any suggestions of Why? I was thinking of adding seafoam to oil or other cleaner! I’ve had vehicle since 1989. Thank you