i tore into my 200,000 mile 93 ranger with 4.0 v-6 to fix what i thought was a bad head gasket. i had some coolant getting into the oil and assumed bad headgasket. well i pulled the heads off and had them magnafluxed and pressure tested. they tested perfect so installed new valve stem seals and re-installed the heads. well i was able to pinpoint the problem being the lower intake manifold gasket had developed an intenal leak.
anyway the tech that tested the heads couldn't beleive how good of shape they were in for the amount of miles on them.