Yes, UTG got it wrong… both times…
The anti drain valve simply stops the oil draining from the oil galleries back into the oil filter can and then back into the sump over night. If such drain back occurs then such would cause a dry start condition when the engine is next cranked which isn’t great (cause assembly lube only lasts so long
).
The spring at the bottom of the filter media is there to seal the upper part of the media cap centre bore against the ant drain valve thus it stops oil entering the oil outlet at the top of the filter as UTG suggests. The oil can only enter the outlet port via passing through the filter media itself or via passing through the bypass valve at the bottom of the filter media. Which path the oil takes depends on oil viscosity and/or pressure. And the filter media doesn’t move up and down on the spring because of the way oil pressure surrounds the whole media…
The filter will go into bypass on cold start but as the oil gets more fluid the bypass valve will close and the filter media will do its job unless oil pressure is too high due to blocked filter media (hasn’t been changed for ages) or wrong oil viscosity (rtfm) or a new high volume high pressure oil pump (which the bypass valve isn’t calibrated for).
Unfortunately there isn’t a video that discusses all aspects but I previously posted the following in the VOTD thread earlier today.
The anti drain back flap is not part of the filter bypass system and the filter media can never slide rearwards because pressure would be pressing on it from all directions.
Here’s how a filter works, first video is how the bypass works and second describes the anti drain flap.
mod edit: I've created a separate thread
here for discussion on this topic.
The following also goes into some aspects…
Really, the filter won’t always filter 100% of the oil flow coming from the pump and that is by design. Just wish they had a oil filter differential pressure sensor and a idiot light and/or DTC and check engine lamp to remind the less mechanically concerned that they need to change their oil filter… Such is QED but meh, save a few cents per engine is better…
PS: I don’t ever remember seeing an oil filter spec define the bypass pressure setting of the oil filter bypass itself in any of the advertising or spec sheet where oil filters are purchased. So one does have to wonder what goes on with engines whose pumps have been upgraded? 50% oil volume filtration?