topping off coolant in the porsche

[equ]

For the first time ever, on one of the colder days (teens outside and mid-30's in my unheated garage), I got a coolant below min message on my 981S. It seemed to go away rather quickly and did not come back for a few more drives and then came back on on another cold morning.

I'm guessing the level is just below the minimum desired by the sensor so on colder starts it triggers the warning that goes away quickly. Guessing it needs to be topped up, but just a bit, so it's at minimum when I check under the cap. It does seem to be below minimum right now. I have a sense that they don't "like" being much above min when cold.

Question: do I top it up with distilled, mix distilled and porsche coolant 50:50 or coolant only? A search says it uses g40 variety.

Car is coming up on 9 years old with just under 28k miles. It is occasionally started below freezing (but not below 0F, or even 10F). I don't think there is a leak; this is likely normal, slow evaporation.

To my knowledge, coolant has not been flushed and I'm sure the more mechanical among you will say it needs it on time. I'd like to postpone that as much as possible. It only does 2k miles/year these days. (Also the only time we lost an engine in a car was when I wanted to be 'ahead' on maintenance and had an indie shop flush a Subie's coolant. Attribute fault somewhere: shop, subie's head gasket, it was going to go anyway, whatever... I'm scared of coolant flushes unless absolutely necessary, e.g. water pump replacement)