6.5hp? Side valve Briggs and Stratton, I can't remember the capacity. I run full synthetic 5w40 in it. It burns a bit of oil. I believe the carb has been leaking gas into the oil since before I bought it used last year.
My previous 1981 Briggs and Stratton 3hp never burned oil. I ran the same 5w40 full synthetic. The engine ran great until I hit a huge bolt hidden near a sidewalk in deep grass and it stopped the engine dead and and bent the rod or crank. That motor had a ton of very hard hours on it, hit lots of rocks, going through deep wet grass, ran for hours straight only stopping when out of gas. My dad ran straight 30 conventional when I was a kid and it rarely got changed because my dad was too busy with other stuff (building the house). It didn't get good maintenance until I inherited it and fixed it up a bit.