RT,

There are only two possibilities:

1) If the orifice is small enough, say 1/64, and the filter is not restrictive for this very small volume, then the pressure downstream of the orifice is effectively zero. Imagine if you removed the filter completely and put a pressure gauge downstream of the orifice. It would read zero. No way around that. The very small flow would simply flow back to the pan. This is especially true if we are using larger lines (1/8 or more). So, in this case, the oil will definitely back up into the filter if the drain runs uphill.

2) The orifice is small, but not 1/64. Say it is 1/16. There is sufficient flow to the filter and the flow backs up at the upstream side of the filter, because the filter cannot accept all this oil. Pressure will then develop between the orifice and the filter. This will cause more oil to be forced through the filter. Equilibrium will eventually be established where the pressure between the orifice and the filter will stabilize and a constant flow will be established through the filter. However, on the downstream side of the filter, the pressure is again very close to zero. Again, in this case the oil will definitely begin to back up into the filter if the drain runs uphill. However, this is the better of the two cases, because the upstream pressure will not allow the oil to back up too far into the filter.

One of the aforementioned options is applicable. I am fairly confident that the oil will back up in the bottom of the filter, somewhat, if it has to run uphill, because the pressure is effectively zero.