Have always used 15W50 (Penrite HPR semi synth) in my car. The 15W is perhaps a bit too 'thick' down south (Ballarat) where I could travel several kilometres before the oil warmed enough to drop down to the 'normal' operating oil pressure of 58psi.

Currently in NW Kimberley where things are quite warm & did an oil change yesterday. Took the car for a drive for a few kms before dropping the oil, as is my usual practice, but found when pouring in the fresh oil that it was a lot 'runnier' than I have found on previous changes. Doubt I needed to take that pre-change drive. I see no reason to change to a 10W oil, at least not until I'm back in the cold.

Possibly of interest, earlier this year down in Tassie I found on several occasions that my oil pressure dropped down to 40psi when towing up some very long & steep mountains, but as soon as it cooled a little it returned to it's usual 58 psi. The fact that the oil had not been 'cooked' & that it regained it's usual properties after getting pretty hot was something I found quite reassuring.