Colder weather means slightly cooler and thicker oil at full temp which would translate into higher oil pressure. Lucas is very thick and also increases oil pressure.
A shop will be able to put a mechanical gauge on and test the pressure and determine of the sending unit is bad, or you actually have low oil pressure.
The vaccuum port where the map sensor vacuum line connects is prone to carbon build up.
If the throttle body has never been removed and cleaned. It is possible the port to the map sensor is filled with carbon, not allowing the Map sensor to read intake manifold pressure and lighting the CEL.
It is not very difficult to remove and clean the TB. The little port on the back where multiple lines run to, comes off with 3 torx bit screws. But there is a gasket which most likely tear upon removal.
I know this suggestion might get Alloro all riled up, especially with you being female and all, but pull the vacuum line off the Map sensor and see if you can easily suck or blow air through it with the other end still attached to the throttle body, to determine if it is clogged and if a TB removal and cleaning is necessary.
The only times I got a code 13 was when my ECM was failing, and causing the engine to sputter, stutter and backfire.
Harbor freight sometimes sells a digital multimeter for $2.99. Regular price $7.99. It's not a professional tool, but I've tested mine against an 85$ Fluke meter and found it to be pretty accurate. It is very worth having one.
Good luck. Is your overdrive working?
Here's a mechanical oil pressure tester for 10$ while your at HF.