I guess I'd never venture into the areas of "thick" motor oil anymore since the 90s, and been staying with anything 10W30 or lighter weight.
Once again, my take is on the specifications and not on any specific viscosities or brands. If a certain gasoline engine grade 10W40/20W50 can pass the ILSAC GF4 spec (which I doubt it, esp. for some 20W50 last I saw on W-M shelves), there's no guarantee on the "cleanliness".
Also: for certain car/engine designs such as VW 505.2 spec., Honda HTS-06(sp?), etc. would be desirable if that's what your engine calls for, in addition of ILSAC specification.
AGain, my take on the subject.
Q.