Many things can contribute to this. The 900 RPM idle speed is on the top end of a fast idle speed in my opinion. The idle speed affects the fan speed and engine cooling. Too low is not good either.
Oil viscosity is also a concern, most Type 1 engines like 20W-50. In the dead cold of winter in Calgary, 10W-40 is a safe bet. Do you drive the car when it’s that cold? Do you use a block heater?
If the motor has a lot of miles on it or was not given regular oil changes before you got it, the oil pump may be on the edge of it’s clearances or gummed up. At speed, the pressure stays great, but at idle there is too much clearance to make good pressure. Do you use a remote filter w/o a high volume oil pump, or a bone stock set-up? Do you have an aftermarket deep sump?
How you check your oil is important too. Don’t let it sit for a long time to let the oil settle before checking it. After the engine is warmed up, shut it off and check it right away. Bring it up to the full level if it isn’t there.
Worst case is a bearing/s that is/are going south, but you would hear that most of the time at idle.
These Type 1 motors can be modified for “Full-Flow” oiling easily. Add a high volume oil pump and a remote filter too. Most every performance Type 1 has this modification, but it a good thing for stock motors too. A deep sump is also a good addition and would not require engine disassembly like the “Full-Flow” oiling mod would (you have to drill and tap pipe threads in the block).
These things are air cooled and depend on oil for cooling, even more than a water cooled engine does.
If I knew more about you engine, I could suggest a better reason for the low idle/low oil pressure. Try the simple stuff first.