I know this is rather the wrong time of year for this, but...

About two months ago I bought a secondhand rotary petrol lawnmower since I
was fed up with the landlord-supplied Flymo with 2" plastic blades. It
seems to be a cheap generic engine-on-a-shell type - I don't see a brand
name. It came with a petrol can, but I forgot to ask about oil.

It worked when I saw it, and when I used it to mow the front lawn, and then
it kept stopping. Noticed there wasn't much petrol in it, so filled it up
with what I assume is unleaded from the can (the seller said he'd give me
the can as he couldn't be bothered to put it in his car). Now it's still
refusing to start. With a few priming pumps I can get it to run for about
1-3 seconds before it dies.

I probably need some oil in there... what oil and what ratio? I'm guessing
it's two stroke since it's pretty basic - but how do I check? Will I have
damaged anything by running it without oil? Now it's refusing to start - so
any tips on getting the oil into the system? Anything else that might go
wrong to a recently-working machine, or that I might be doing silly with it?

I suppose I could strip it down at some point, but that sounds a bit too
energetic for the moment. Meanwhile up until the last week or two the grass
has kept growing

Thanks
Theo