I have a rebuilt L98 that I have run-in first with running in oil [1000 miles], then used regular mineral 20W50 oil then at 5000 miles I switched over to Mobil 1. The engine is definitely mechanically noisier although there are no distinctive knocks or other real scary sounds. It does not sound like there is just 1 or 2 rod/main/cylinder that is noisey, rather the entire engine sounds noisey.
I readjusted the valve lash which removed the slight lifter noise but the bottom end is definitely noisier than before. In Australia, Mobil 1 is 5W50 and the temp in the winter here on the West coast is normally between 10 and 20 deg C [not really cold!].

Is 5W50 too light for an L98? I blueprinted the engine and got all the main clearances very close to 0.0025" and the big ends are all at 0.002".
I have good oil pressure [about 55psi hot at idle].

Has anyone else experienced this? Am I dreaming? If I have good oil pressure, there should be no metal to metal contact even with the thinner oil should there?

 

5W50? Yes, that may be a little thin for your engine. I use 15W50, and 10W30 when the temperatures are expected to range from -40° to 0°.

Your weather should star getting warmer soon, so plan on something with a bit more viscosity for the next change.

 

Try hard to find some 10w30 Mobil1, they make it so it should be available, I've run 5w30 Mobil1 in my car when they were sold out of 10w30 and didn't get any noise but I usually run the 10w30. I would say you may have a couple bearings with more clearance than you think they have. Stay away from any 50w oil, it's just way too thick IMHO.