Thanks for the kind words Werecow. I also want to say, im learning everyday just like everyone else. Any info i know is info you will know.
Bronze is oil impregnated and doesnt really secrete unless its properties are compromised....or something like that. With that said its another reason for the use is bronze ability to wick heat from the valve stem and transfer to the aluminum head.
Funny thing about powdered metal, its heat expansion is minimal compared to the material the valve stem is made of. Meaning the stem will expand them clearances smaller.
Also, i dont know of any modular head that had in tolerance valve guides. Whoever set the tolerance must have been looking at manufacturing blueprint cause any head ive ever checked, the tolerance was either borderline out of spec or just simply out of spec. And the heads came from perfectly good running engines. None ive ever checked were considered acceptable by any builder, but they all run good.
Also the valve side play is exactly the same any direction when checked. Where as the guides get worn typically from stress pressures from the rocker/follower.