The actual PRESSURE of the oil hardly matters at all.
What REALLY matters is that the oil is circulating all the time, especially through the main bearings of the crankshaft, then the big ends, and the camshaft shells.
As long as the wee light is OUT you think there is pressure. Of course it could be that the little bulb has blown, or the wire fell off the pressure -switch!
If you are diddling about a field popping bunnies, and the wee light comes on, don't panic. Blip the throttle, and if the light goes out you know the oil is circulating.
A slightly heavier grade of oil might help, and in that model I suggest tractor universal 15W 30 as ideal. My TD5 runs on that, now at 48,000 miles.
CharlesY