I'll give you something to think about---We run some 500 inch drysumped engines that have about 20-25 inches crankcase vacume thru out the rpm range all the way to 10,000+ rpm.
If we removed the pressure pump COMPLETELY out of the system, the engine would run just fine with the vacume sucking the
oil out of the tank and thru the bearings---THINK ABOUT IT--- just like drinking a malt threw a straw
Jerry