Thanks all!
Let me clarify a couple things: *I* put the 0W-20 in the truck after the *new* lifters started clattering on full syn 5w30 (GM recommended for truck). The old lifters clattered on whatever was in the truck from the used car dealer, which is why I took it in.
When the new lifters went in, the intake gaskets were replaced, and there's been no lost (or gained) coolant since.
I figured a thinner oil would get past whatever was stopping the oil from getting to them when cold, and that full syn would start dissolving whatever buildup was in the galleries. It worked.
20 weight oil is only about 5% thinner than 30 weight (BITOG, oil univ 108). It ain't damaging the engine. If I were working the engine hard in hot weather it would - I'm not, and haven't. When the oil pressure dropped during the move, the operating temp did also(to the tune of 10-20 degrees).
I'm with TFB1 and jstutz: if there was no oil getting to the parts, the lifters would make a racket, so the sending unit is lying to me.
I'll check it, and post pics (if I can figure out how) when I get the valve covers and oil pan pulled. The truck's in a holding pattern while we move into our house... have to get unpacked so there's space in the garage to work on it.
I've been pretty deep in an engine or two, but never had to deal with sludge before (used a solvent tank to clean the parts). I don't want a solution in a bottle, but am looking for ways to 'hurry things along', so to speak.
Again, thanks for the advice.