when is a/c pressure too low

Last fall I noticed a small hiss when starting the a/c. I looked around the system and noticed a thin film of oil/dirt around the high valve. I guessed my high pressure shraeder valve to be leaking so I stopped using the a/c as I had read that an empty system can attract moisture and really bad/expensive things will follow.

I finally tackled this problem today. I replaced both valve cores using a device that keeps pressure in the system. As far as I was able to tell, no coolant was lost during this removal. After attaching the manifold gauges to the system, I discovered my psi on the high side is around 37 (car off, temperature around 40F, humidity 50%). When I tap the valve with a screw driver, a hiss can be heard so I am assuming my system is still under pressure, even if it is low.

Questions

1) I’ve read that if a system is empty, one needs to apply a vacuum to eliminate any moisture. Moreover, if the system was empty for a while, one should replace the receiver/dryer. Does this advice also apply if the system is low but not empty?

2) I’ve read that system leaks on the high side don’t leak much oil. Is that true?

3) I’ve read not to charge a system below 55F. Is this because of the difficulty in estimating the correct amount to add? Would it be fine to add just a little to move my system from low to medium and raise it to 100% once the temperature rises above 55 degrees.