Ignore the man behind that currtain!! (ignore my first post- I was being funny - a crippling affliction
)
Ramblin, you've got some interesting conditions there. I can't hear your noise ...so it's hard to say.
You indicate that this goes away ..or rather gets reduced after a few days with a 40 weight (Rotella 5w-40)
You indicate that the sound decreases with mileage. Now I've seen this happen it's almost always been HLA/lifter noise ....which you've ruled out already.
In my warped mind's eye ..I would go in radical polarities with viscosity. Now you say that viscosity isn't the determining factor ..but not all 30 weights are created equal in viscosity.
Right now, my money is on the visc being too high. This is the only thing that should be getting reduced in that short an OCI.
This you will know with the 15w-40. If the sound is WORSE with this oil then it was with Rotella synth ...then you know that the Rotella produced less noise due to being a synth ..and not a 40 weight ..and that some mechanism/process further reduced its viscosity in a short time ...just as it does for your dino 30 weights.
You mentioned that the 5w-30 was the loudest ..but you also indicated that it was use in the winter. Does that mean that your 10w-30 was used in the warmer months ..and that you DID NOT use the 5w-30 in warmer months?? This could be the difference in your perception, if you see what I mean
If it was my ride ..and I didn't mind potentially wasting some good oil if it didn't work out, I'd throw in some MC 5w-20 and see if the condition is relieved. If it went away ..then it's a weakening oil pump. It may give good indicated pressure at the filter ..but cannot produce the "head" to reach everything until it's thinner.
Any UOA's on this engine/vehicle?? That would probably show the mechanism that is allowing this noise to reduce as you approach the end of the OCI. My money is on some shearing/dilution effect.