The B engine oil pump (being external) is simple to replace. A worn one after long period of non use could have loose enough clearances that suction may be lacking. I would get a quality, blueprinted high volume pump and see where that takes you. Relatively inexpensive and non invasive. Maybe reevaluate oil level in pan and type of oil used. Definitely change oil after running and try to prefill filter as much as possible. I run HV pumps in both A and B motors and have had no problems.
A quote from a published source...
[h=3]"When making recommendations to customers, I use the following guidelines. If the customer has a rear-sump oil pan—where the deepest part of the pan is toward the back of the vehicle—and the oil pump is in the front of the engine, with that big, long oil pump pickup tube in between, I would recommend that a high-volume oil pump be used."[/h]