I was advised years ago to wear long sleeve shirts in very hot weather to help control overheating ..is that true?

Don't think so: I think it would seem to make it worse.

Yes, but...: Yes, that's true. Long sleeve shirts & pants will help in hot weather if it is thin, breathable (preferably cotton), light-colored clothes. This will keep the hot sun off your skin, perspire less, & lose less water. For example, in equatorial africa and the middle east, they normally wear long sleeves. This is not just cultural but a good tactic against the hot sun. Cover the neck & head too.