If you have not yet changed the oil cooler gasket, better to do it ASAP.
All Mark IIIs are almost two decades old and this ageing gasket is like a time bomb.
When it fails, engine oil just pours out from the base of cooler.

You can use old oil filter gasket instead of buying a new o-ring as it fits properly.
You need 27mm socket, a 3/8" internal pipe extruder to complete the job.

Again the mighty youtube has several very good video showing you all the details.