Pressure Drop - song - was created in 1970.

Use a delta-p gauge

Pressure Drop - album - was created in 1975-11.

No. The pressure drop remains the same. The only thing that could led to a smaller pressure drop after 1 (or more) year, is the internal surface friction coefficient. But this coefficient does not change in a measurable way. Perhaps small particles are re-arranged inside the pipe, but the coefficient remains the same. So does pressure drop.

Yes, low oil level can cause low pressure.