Check the thermostat housing (gearbox end of engine) as well these often warp/crack and cause a coolant leak which can often be missed as it tucked down the side of the engine. Also worth removing undertray if fitted and parking the car over some paper/card of similar and looking for coolant and other stains on the paper to help locate the location of the leak.
Lastly not sure if the petrol engine has a oil cooler if it has these can also be a cause of a coolant leak (I had this on my DS)
have used Wynns Hydraulic Valve Lifter, full bottle for a month then fresh oil/filter change with 1/4 bottle.
your oil will go black as it cleans, keep a eye on it.
carlube oil treatment also works slowly.