I have been told not to use heavier oils like 10-40, 5-50, or 15-50 in the newer engine's that are supposed to use 5-20 as they have very close clearences and it will damage the engine's because the oil is too thick. Any truth to that?
IIRC Ford back spec'd most of their engines back to 93. There are certain exceptions like the 4.0L which still calls for 5W30.
There was a link to all the engines that were back spec'd, but I can't find it for the life of me.
In the Ford 4.6's they used to spec 5w-30. Then changed to 5w-20 because they needed better CAFE standing. However, the specs and clearances on the engine didn't change. For this reason i run 5w-30 instead of the 20 the manual recommends.
I tried using 5W20 in a 93 Aerostar that was back spec'd to 5W20 from 5W30, and actually saw oil use drop a little. I'm now running a mix of 5W20 and 5W30 to clean up some left over qts I had laying around. I will run 5W20 again this winter. I was very pleased with the 5W20 oil in the old Aerostar.