In the early days of the car wash i had a ford ranger, and a jiffy lube next
door to wash. I changed the oil at JL every 5K and made sure they used a different brand every oil change. I bought it at 85K and sold it at 230K. I motor was the best part of vehicle left. So i should have changed the oil half as much, so the motor would have quit the same time as the rest of the vehicle.
On synthetic -many use the synthetic in the winter on snowblowers as the oil stays thinner-therefore easier to crank over by hand. My one car has free oil changes from the dealer every 10K, and it is synthetic, and the oil is still fairly clean.