Vern,
This is were my query started from.
Going back to basics to try and explain what I mean, oil filters have at least 2 valves. One is the rubber flap seen beneath the 6/8 holes where the oil enters the filter and this is an anti drain valve and the other valve is installed inside the filter seen at the bottom of the central hole and this is a by pass valve that opens only if the filter gets clogged to avoid oil starvation to the engine. In the case where the oil filter is installed facing down , as in the P6 4 cyl, usually a second anti drain valve is installed at the top of the central hole where oil exits the filter. In the case of the P6 (and there are other applications) instead of this 3rd valve, a tube is installed to the bottom of the central hole and this also is an anti drain valve (the standpipe).
Now to my original query. The 3 rd anti drain valve is spring loaded and will require oil pressure to open (I do not know how much pressure is required ) meaning it will restrict oil flow. The P6 was not designed with this spring loaded valve but with the standpipe that does not restrict oil flow. Should I worry about this oil pressure difference?
In the meantime I have carried out an oil change and installed one of the Technocar filters I have (these have the spring loaded central anti drain valve) and have not seen any difference from the other filters used to date. The oil pressure light goes out on start up within the first 2/3 seconds. I cannot say exactly how much because I am usually pushing in the choke to avoid revving the engine when cold.