I have a gauge like this one on my WD45 and it runs between the R and M in normal.
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The pressure the gauge reads is the resistance of the engine oil flowing thru the filter. Spec is 12-15 psi maximum. An incorrect paper filter can read very little or near zero pressure. The proper filter is a cotton string filter. A dirty filter will show more pressure than a new filter. If the pipe stem is missing the pressure will be very low. If the pipe stem is pushed clear down to the bottom it can also show very low pressure. As long as the needle shows something, you have oil going to the engine components. The oil that is being filtered is excess oil not being used for lubrication of the engine. I can assure you as a young teen I have seen WD-45 oil pressure readings of barely "N" while plowing or discing, and the engine never failed from lack of oil pressure. Whenever one of these engines is overhauled, new camshaft bearings are the foundation to good oil pressure when you get all done.
 
The A/C engines of that time used low pressure oil systems in them. That said if you have the wrong type oil filter on then your pressure will be way to low and can in turn cause engine damage. If I remember right I use the Wix 57101 but to be 100% sure on that number I would have to go out in the snow and look at one of my A/C tractors
 

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