Simon, I have a nodular cast iron cover on the rear diff (5/16" wall) and you can get heavy (5/16" and 3/8" wall) plated steel ones too.
They are heat retainers but protect the CWP very well and are needed for when you shave the bottom of the housing to get some clearance.
heavier oil is a work around for the retained heat. Bit of a catch 22 as a heavier oil runs hotter anyway from the increased drag, but it helps protect a hot running Sals.

