You always recommend compression tests.
I googled and it sounds complicated and as far as I know, no one has ever
Basically, a compression test is used to find out if there is wear in the
piston and rings and also if any valves are leaking.
The test can also indicate if there is a blown head gasket.
It involves removing all the spark plugs and using a compression tester
to take the pressure in each cylinder by cranking the engine.
The test is repeated after squirting some engine oil into each cylinder
and cranking the engine a few times.
If in the second test the pressures rise appreciably, then it shows wear
in the pistons and rings and if there is no improvement in a particular
cylinder, it may indicate broken rings.
These are some of the conclusions that can be made but a good mechanic by
using a variation of these tests can check out for other faults.
If your mechanic does not know how to do a compression test or does not
have a compression tester, perhaps you need to find another one.