Piston slap is very annoying.  RESTORE does a good job on cushioning piston slap providing the cylinder walls haven’t worn too far and the bores have not gone too oval.  Try 100ml RESTORE down each cylinder (spark plug hole in petrol engine - injector hole or glowplug hole in diesel). Explain to your mechanic you want to do a dry/wet compression test only using some RESTORE OIL instead of ordinary oil.