I did some research for a post and compared the main bearing clearances on a Chevy 350 (which spec's 5w-30) and my Ford 4.2L V6 (which spec's 5w-20). The ford's bearing clearances were smaller. I don't remember the specifics, but it was by about 10-20%. I realize that these are two vastly different engines, and I also realize that Ford spec'ed 5w-30 for these engines for four years prior to the 5w-20 release. But if the FOrd engines that have been spec'ed with both 5w-30 and 5w-20 have main bearing clearances that are tighter that a motor that is spec'ed with 5w-30 thru 20w-50 (?), than I would be hesitant on running 5w-20 in that chevy.
My point is, if you can get your hands on a couple manuals with the bearing clearance for each motor (both your Ranger and Kia), compare the two and also compare the engines. If the engines are close in terms of clearances and tolerances, run in the same temperature range, same duty, close in head design and so forth then go ahead a try running 5w-20 with an UOA. If not, than stick to your manufactures recomendations.