Mk 2 Carrier water can

[Grant Bowker]

I agree with the 80 being more likely octane than oil viscosity.

Consider that many cars now use 0 weight oil for fuel economy and ease of cold starting. In the WW2 period 30 weight was fairly common. I've never owned a vehicle tha called for anything heavier than 20W50 (motorcycle and 1976 VW and the VW only called for 20W50 for sustained very hot weather driving, mabe a holdover from the aircooled Beetle?) Oils in the 75 and above viscosity range are more often gear oil for transmission or axle use.
In that period, 50 octane gas was still in use, 70 considered regular, 80 a great improvement 100 for high powered aircraft engines and I don't think 130 and 145 aero fuels came along until post war. To further confuse things, fuels were often dual rated (eg. 80/87 or 100/130 for lean and rich mixtures - more commonly seen when dealing with aircraft where the pilot can directly control the mixture).