ol thumper

Hi all, I've just joined up. About a month or so ago I bought myself an ol' XJ900 for $400. I put new tyres on it, changed the plugs and got it registered. It goes like a scalded cat and I noise it makes when she's barking along. A Anyway, I have noticed int he last couple of days that the oil light comes on periodically. Knowing a bit about mechanical stuff I know it to be a warning light as it progressively stays on longer. What I don't know is what to check. I don't have a workshop manual to refer to so that's what brings me hear. What do I check and where? I'm taking the suck eggs approach and would rather learn from someone more experienced in these old girls than with guess work. looking forward to hearing back from you, great site, heaps of good info, keep 'em on the black stuff!! Jake