I converted mine from dry to wet sump, as I didn't have an oil tank. I just poured the oil through the opening created when I took the side door off until it was at an appropriate level. I had already made a dipstick, so I marked it at 1, 2 and 3 gallon levels for checking purposes. Initially I put in 3 gallons, but as I put a squirt of oil on the valve gear every day, I now fill to 2/12 gallons and the daily squirt from the oil can gradually tops up the sump.
What oil are you using? I used to use Morris's, but I now have a paper oil filter in the sump so that I can use cheap ( well, not really, perhaps cheap-er) Halfords 20-50.