How long does it take the oil to drain to the pan and read on the stick?
20 seconds tops. Most accurate reading is 15 minutes after shut down when hot. But for aiming for that cross hatched part to get covered, a few seconds is all it really takes to get a good enough reading. If you hit the bottom 1/4 of the cross hatched on the dipstick then you have enough oil to be safe. Covering the cross hatched area when cold will overfill the crank case because the dipstick in these trucks is calibrated to read accurate when hot (10L of oil can expand a lot when at normal operating temperature, hence the wide range on the dipstick).
Sorry, i should have explained that a little better. I drove the truck for 20 minutes total at which time the gauge said passed half towards high, then it started to flick around, and then was at zero, and the engine light on, i was about 3 minutes from home when this happened.
OK, that sounds a lot better, whew....
The engine light came on at the same time you lost pressure? Fill the oil all the way, but its possible that its just a loose connector at the pressure sensor. 1987 is not actually capable of reading low oil pressure even though it has a gauge. The sensor is actually just a 7 psi on/off switch, so a low reading can only be an electrical problem in most cases. If you consider what I said earlier about oil expanding when its hot, it seems unlikely that oil pressure would be fine when cold, then cut out when hot because the level would actually go up slightly.
The oil pressure sensor is located right at the back top end corner of the engine if you want to have a look at that connector. I bet its probably a little loose.