I Live in Kelowna BC Canada, and it just hit -10C, i have a 88' Sierra Cosworth with a recently rebuilt engine, New 200 bottom end, and 2WD head (rebuilt) and it has 3000 miles on it.

I am using Castrol GTX 20-50. but like i said it hit -10C, what other grade of oil can i use given my engines low mileage?

And when can i start running fully synthetic ?

Thanks