Seen this marking a few times and it's on my 1987 GTA as well. The hand written -30* C

What does that mean? Did a search but the closest thing I could match was -30 pound injectors.

 

My best guess is some shop in your area flushes the coolant and writes that on there to show the after-flush engine coolant freeze point.

 

Well I'm in the Miami Florida area so I hardly think anyone in my area put in a -30 degree antifreeze.....

...however, while I bought the car in Miami I know it was once owned by someone in Minnesota. So that makes sense.

That picture above was taken from an ebay listing and it says the car is in Mesa Arizona. Does it get that cold in Arizona?

 

-30* C is -22* F, that makes sense for a warm climate vehicle because a higher freeze point means a higher boiling point as well. A 50/50 coolant/water mix would give approximate -37*C (-34*F) freeze point, but slightly lower boiling point as well.

 

But is it normal to make that marking? I have owned a lot of cars and trucks, most of them used and I have never seen this marking on anything but a few 3rd gen's.

 

I've seen it a lot over the years. When I worked at a dealer we saw it all the time from one specific chain of tune-up shops. The kind of place that would sell plugs, wires, filters, and flushes then send the customer out the door $500 poorer and still have the same misfire they came in with.