97 ford f150 has a 4.6L with 166,000 miles and runs strong. how is it possible since i have ran rotella in the motor from the day i got the truck new in march of 97. i am going to school in ohio and might think about putting Amsoil in it. would you change oils if the truck runs good and never had any problems with anything on the truck. oil changed every 3,000 miles. what could be long term damage from the heavyer oil in it.