I started driving in 1960, well 1958 if you count the motor scooter. I used Pennzoil back then, you know the kind people used to say caused sludge. Any way I used either Pennzoil or castrol GTX for the next 40 years with absolutly no sludge what so ever. But I have always maintained my cars and trucks really well. I have used synthetic for the past 10 years.
Now I have always used synthetic in my 1995 Cub cadet. It has a Kohler engines and runs as good as it ever has.
Just my experience