after finishing the 940 low oil pressure job I think I may have learned why they fail.... apart from obvious tear.

my daughters husbands 93 940 had the classic o ring out of the oil delivery tube that enters the block.
the o ring on the tube in the oil pump was perfect.

the block o ring was not torn but was pushed out over and out the lip on the tube for about 50% of the diameter of the o ring. clearly this had been like this a LONG time.

when I unbolted the oil pump and wiggled I found a slight amount of play in the delivery tube which disappears once the pump bolts are tight this allows the block part of the tube to be fastened tight in the hole but not CENTERED in the hole once the pump bolts were snugged. I found by wiggling the delivery tube to center the tube in the block hole the amount space around the metal lip easily became uniform. when this potential play is ignored why snugging up the pump bolts the default position of the tube is slight off center allowing more space on nearly half of the diameter of the o ring which corresponded EXACTLY to the area of the ring pushed out and over the tube lip.

therefore it is my conclusion that centering the delivery tube in the block hole WHILE snugging up the pump bolts is absolutely essential to avoid redoing this job in x miles.

i reused the original volvo oil pump and the system now measure robust spec oil pressure. a hump of a job which cost 2 o rings and a pan gasket....10 bucks maybe!