Usual procedure with tiller worm drive gearboxes, is to fill until the oil level is at the bottom of the filler plug hole.
Unless it has a second level plug fitted, which should be obvious if it's there.
No idea on the quantity required, but you'll probably have to buy at least a litre anyway.
Usual oil type is a gear oil in the SAE90 to 140 range. But as the pinion is 'yellow metal' [i.e. bronze or brass] in these, from a past ODK thread, the oil must NOT contain sulphur-based Extreme Pressure [EP] additives. These are corrosive to copper alloys.
So common automotive driveline oils are not suitable.
Penrite's 'Transoil 90' or 'Transoil 140' would be about the easiest to get, in a smallish pack size [1L]. These oils are formulated for vintage car drivelines, made before the advent of the 'hypoid' design of diff gearset.
Edit: There is some evidence that these tillers were made by MTD, though I can't readily locate an exact user manual. Nonetheless, all of the 1980's MTD worm drive models I've found used SAE140 oil, API GL-1 Spec. Which those Penrite Transoils meet.
These tillers were a Victa rebadge, probably of US origin. It would be necessary to work out what the manufacturer and their model designation was, to find more detailed info, as there doesn't seem to be much about under the Victa name.
Last edited by Gadge; 16/05/15 06:41 AM. Reason: add oil spec