How old is the engine? Hi time or low time? Is the oil diluted from fuel
getting into the sump? Does your gauge show in PSI or just a green and red zone? The first thing I would do is change the oil and see if that makes any difference . Make sure you are using the correct weight and grade of oil, and see if the oil you drain out of the engine seems to be thinner than what you are putting back in, or if the used oil smells like diesel
. Use an actual Volvo oil filter
even though they cost way more. If your oil gauge doesnt show in actual numbers, install one that does and see if the low pressure is within the manufacturers tolerances . You may find that you dont have a real problem. Since you didnt say anything about engine temperature, I assume that is not the problem, but if you only have a over temp warning light you might be running on the edge of overheating
,which could cause the low oil pressure. Give us some more info and you will get more and better advice
. Hope this helps.____Grant.