My boss owns what I believe is a 2006 H2/ 6.0L hummer. Last week he said the lifters started making alot of noise. He took it into the dealer. They did an oil pressure test on the top end of the engine and the bottom end. He was told the pressure was good on the bottom end and low on the top end. I've never heard of that. Does any know what the cause may be.
He said he changes the oil regularly and uses fully synthetic oil.

 

Possible causes that I can think of are

Bad oil pump
Sludge restricting oil from reaching the top end

Personally I would start with replacing the oil pump and simply flush the motor at the same time....

Disclaimer: I am not a mechanic.

Check with Muddydawg75 he would know more than me