Tried SeaFoam ?

that stuff works wonders for cleaning engine oil passages
and sludge out of an engine.

When I tore my stock '89 100+k 5.0 engine apart for first time
I had been really bad about changing oil, car had sat for 2 years at one time
but I ran a couple of cans of SeaFoam thru previous oil changes as I started
taking care of the car again.

When I broke it open, there was not a single bit of sludge or old oil build
up ANYWHERE. under valve covers, oil pan, everywhere looked super clean.

True believer after seeing that. Was expecting to see ugly mess under v/c's