There will be quite a bit of air in the mixture, and over time it will shrink a bit.
Some things are not worth modelling, just get some materials in and start mixing.
Now I assume you are thinking of a standard imperial oil drum (45 imp gal) as opposed to a US oil drum (55 US gal, or 46 imp gal).
Or somewhere between 205 and 208 litres.
I used to listen to 208 MW when I lived in France, was one of the only English speaking music channels we could get.