Hi Everyone,
Driving back from Scotland yesterday and got a check oil warning message. Pulled into services and popped bonnet to find grey/black sludge coming out of the expansion tank and no oil left on the dipstick mark

Put 1 litre in and finished my journey

There is no water in the oil , no residue in the filler cap or dipstick, the car does not overheat, in fact if I hadnt got the check oil message I would have been none the wiser. Have searched the forum and it seems probable that it is the oil cooler, but would like someone older and wiser (well wiser anyway) than me to give some advise.
Cheers