My M20 has had its oil cap (as well as tool box knob and near side steering adjuster knob) painted red at some time in its life. Was this a wartime practice or was it done later?
At some periods the British Army did apply red paint to certain vehicle parts, split rim wheel nuts on trucks are a famous case. Its more likely to be post war but it could occur at any time that a unit had too much time on its hands and an over keen CO.
My Ariel wng has a red painted petrol cap. I assumed it was a guide to the less mechanically minded (see that red thing there laddy? That's where you put the funny smelling liquid!)
Red fuel caps (and jerry can tops) became common in the 1970s for petrol engines, with yellow for diesel when the two fuels were widely in use alongside each other. Diesel engines were quite uncommon in WW2 (Matadors and Scammells ?) so it was not usual to differentiate at that time.
In general with a WW2 vehicle, the less painted embellishment and universal pouches strapped to the forks, the more accurate it will be
