For years, I changed every 3k miles and used cheap multigrade oil or Exxon (since I worked for them in lube oil industry). Beheaded a Fiat once, and it had NO cylinder bore wear ring that I could feel. So, this keeps wear down in the lower end of the engines. Now, since retiring and EM merger, I put 0w30 Mobil 1 in every engine I own (because Exxon tests had shown it to be the best). It's convenient, as I can run all year without changing. Each vehicle runs about 6-7k miles/yr, and the tractor goes 60-100 hours/yr. The generator sometimes goes 2 to 3 years because it's a pill to change its oil, and it isn't run much. The patch of grass where I drained it last year is still bare.
One of the engineers who used to work for me quit and went to work for Sun for a while; he's back with EM now. He said Sun did a taxicab fleet test, some with dino oil and others with Mobil 1. The ones with Mobil 1 had NO valve train wear. This is how Mobil 1 gives you 4% better fuel economy and hp; hp isn't being spent grinding away some valve train components. I've experienced 3 to 4% fuel economy increase on the 4 on road vehicles I've put it in. Can't tell on the tractor.
Ralph