My understanding of the 20-50W bit is, it's a 20 weight oil that protects like a 50 weight.

Same deal with the 5-30W.

Before I swapped over to synthetic in my (new at the time) 2002 F-150 w/5 liter I asked about 5-30W vs. 10-30W - both synthetics - on a board that's run by a well known tech writer.
He said - in so many words - that the 10-30W will give better protection.

In my opinion, manufacturers recommendations are not always for the consumers best interest, but for the corporations best interest.

Keep in mind the 5W oil debacle on the Buicks a while back.
Quite a few of them wiped out bearings and not too long after we find that the 5W recommendation was to better the CAFE bit for the company. (CAFE = Corporate Average Fuel Economy.)

Following the manual is the way to go in most cases, but you have to do some thinking for yourself as well.