1988 Ranger oil pressure light

I have a 1988 Ranger with about 130,000 miles. I've only owned it for about a year, so don't know much about the repair history. I do know that the fellow who owned it before me was very careful about keeping up with maintenance, oil changes, etc.
My problem is that when I start the truck the oil pressure light stays on for a long time. I don't know if I'm going to damage the engine because of this. Once the truck is warmed up the light goes off and the problem doesn't re-occur unless the truck sits for many hours (ie: overnight). The problem is getting worse as the temperature gets colder. In summer it went off in about 15 seconds. Now it's going on and off for about 5 minutes.
The truck runs well. There are no funny sounds, smells, etc. It does use a bit of oil. I have to add a quart after about every third gas tank fill-up.
Any suggestions? And most importantly, am I likely to be doing engine damage? I don't really have the money for a repair right now. Hoping this might be able to wait a few weeks as long as I warm the truck up well before driving. Thanks!