Hi all, replaced head gasket as the start of small amount of mayonnaise in the engine. Obviously have drained the engine oil out, have yet to remove the old oil filter (which was only a few weeks old). So now is the time to start the refilling of the fluids. Should I buy some flushing oil (5 ltrs) and fill the engine and run for an hour or so (with the existing old oil filter) and then drain, wait overnight and then change the oil filter and refill engine with the correct oil specification? If so, any recommendation on the flushing oil to purchase / use?

Secondly, the cooling system had small sign of oil in the water expansion tank. Should I just flush the radiator and water pipes with clean water and then reverse flush the radiator, or should I fill it up with clean water and add an additive (if so what?) and leave this in the running system for an hour or so and then drain, refilling with the 'red' OAT product.

Advice appreciated.

Well in the past I have used a litre of diesel and 3 litres of a cheap engine oil to flush out the lube system. I used the existing filter. Drained and then I refilled with straight engine oil - again a cheap one. Then a drain and refill with a good oil and a new filter. I left it draining overnight and jacked up the rear a bit to get that last dribble out of the sump. I just had the motor running in the driveway on idle - didn't actually drive it around. Just left it idling till it came up to temp' 10 - 15 mins? then drained... It's a old mechanic's tip and seemed to do the trick too. There may well be a modern chemical oil flusher available, probably does a similar job.

Cooling system flush: I use ordinary domestic [kitchen] citric acid powder. Just a small pack mixed with warm water to dissolve. Drain and fill system with it and again left the motor idling in the drive. Drain and flush out with plain water - disconnected IRD cooler pipes, heater pipes... flush as much out as I could. Used a gentle air line to blow through pipes... then refill using OAT and rain water / distilled water. The radiator is the trickiest bit to flush out. Long ago my HG must have gone. I replaced my rad last year. I took the side tanks off the old one and was surprised to see a greasy oily deposit about halfway up the core and right through the tubes. My critic acid flush didn't 'dent' that. You could remove the rad and flush through with meths or something similar to see if any dissolved oily residue comes out. Meths will evaporate quickly and shouldn't cause issues with plastic bits - will wash out with water too.