Generally just normal wear and tear. Also if a rocker arm is too loose it may cause abnormal wear, loose hydraulics usually have a loud clicking sound to them. Another cause could be a plugged oil galley from the cam shaft bearing, if this happens, normally you will not have oil at the valve rocker arm.
That would be the flat tappet cam in that engine.
If it has center hold-downs on the rocker covers, it's probably a roller cam.
you can use stock 350 plugs
More information is required to answer this question properly. The best way is to pull the casting # off of the block. Is it a truck 350? car? year? roller? flat tappet?
Not sure, i would say oil pump or cam bearings
The camshaft of a 350 Chevrolet engine is held in place by a retaining plate or the lifters. The retention method depends on whether the cam is a roller or flat tappet design.
The 2009 Lexus IS-350 has double overhead cam (DOHC).
The 2012 Lexus IS-350 has double overhead cam (DOHC).
The 2013 Lexus IS-350 has double overhead cam (DOHC).
The 2006 Lexus IS-350 has double overhead cam (DOHC).
The 2007 Lexus IS-350 has double overhead cam (DOHC).
The 2010 Lexus IS-350 has double overhead cam (DOHC).