A relative of mine once wiped a rod bearing on that engine. The bearing that failed was the one furthest from the oil pump, indicating lack of oil pressure was involved. It didn't have the famous melted intake manifold either.
I don't know why the oil pressure got low, and I'm sure it wouldn't have happened under normal circumstances. But that episode makes me shy about the idea of using thin oil on those engines.
I've noticed GM put "10w30" on the oil cap (at least in 1997), while they normally put "5w30" on the oil cap of every other engine I'm aware of, dating back at least to the 80s. The somewhat "unique" labeling for that engine makes me wonder what they were worried about.
To use up the 5w20, you could mix it with some 15w40 or 20w50 to get it up to a 30 grade.