Change out to 90w (old Canter truck)

[Spoonydan]

I'm full time travelling Australia in a 1990 4x4 Mitsubishi Canter truck light truck. A number of months ago I did the 5sp manual fluid. I used this product as per their lube guide.
https://penriteoil.com.au/assets/pis_pdfs/PRO GEAR 75W-85 (Full Syn.).pdf

A GL4 full ester syth 75w80. It was far south and cold then (remember we're upside down ) As I'm now far north and warm with only getting warmer I wonder if I should have went a straight 90w. Very short geared and lives its life in 5th (which is weak).

Keen to hear any thoughts if keeping the high end fluid would offer enough protection for warm sustained highway use, or if a 90w, even if it's mineral be better. Is thickness at sustained temps most important with gears?

Cheers
Dan

Oil data below
TYPICAL DATA
Colour Amber
Density at 15°C, kg/L 0.873
Viscosity, Kinematic, cSt at 40°C 77
Viscosity, Kinematic, cSt at 100°C 12.0
Viscosity Index 152
Viscosity, Brookfield, cSt @ -40° 111,000
Zinc, Mass % 0.139

 

[Kira]

Does their lube guide specify any other grades?

 

[oil pan 4]

That should do it. Usually they're well over a hundred cSt at 40c.
How cold does it get there?

 

[Spoonydan]

That local oil producer only recommends that fluid/grade, some others recommend 80w

For the next 9 months temps won't be lower than 20c (68f) at night and will be getting above 30c (86f) during the days

 

[Spoonydan]

Couldn't work out how to edit the original post, I wrote 75w80 when it's a 75w85.