[Schrade]

Put in a quart of 20-50, and see if it maintains proper pressure. If not, try a second quart of 20-50. Be sure to draw off a quart each time.

If so, you might have too much wear in the crank and connecting rod journals. Change the oil, and go with half 20-50/half 30wt oil.

If it still drops, you are probably in need of a rebuild, short of using straight 20-50 oil (or gear oil).

There's always the possibility of a bad reading. Is the gauge mechanical, or electrical?