Basically use the lightest weight oil recommended for your engine clearances and intended use or abuse, as it may be.
Also there is not one reason to use synthetic oil over fossil oil than to make the owner feel all warm and fuzzy inside and help them sleep at night. It'll just make you poorer, faster.
A use of a proper maintenance schedule is more important, the fact that you change your oil is more important than what type of oil it is.
BTW if you simply want to piss this away as the ramblings of a know-nothing pencil pusher so be it. However I will have you know that I've worked on damn near anything that can roll, crawl, climb, haul, plow, fly, dig and cruise around so this comes from experience, take it as you will.