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IMO, your oil choice is OK. Don't know if Mobil-1 is your synthetic, but their 5W30 actually has a higher viscosity at high temperatures than their 10W30. Owner's book recommendations of 30 years ago don't really apply to modern oil. (Oils for flat tappet/pushrod engines is another story, but Volvo hasn't had one of those in a car since around the mid-70's)
The owner's manual for my 2016 XC60 T6 calls for synthetic 5W30 only and that engine has a lot more stress and high temperature conditions than your 240.
CB is correct on the most likely cause of your delayed oil pressure. You need a filter with an anti-drainback valve (one that works). The preferred choice of most people here is the Mann W917 which was the filter Volvo put on at the factory and which dealers used - when they serviced 240's. Available from our usual internet suppliers.
Another good choice is the WIX 51311 (sold by NAPA with their logo as 1311).
Harbor Freight sell an oil pressure test kit which removes all doubt about the warning light sensor and tells you what is really going on. Item 62621 at $22.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.
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