Dear, it's not only temperature that measures the oil usage.
1 - Yes for temperature point of view at least your lower and higher limits should be according to weather.
2 - BUT oil with lower viscosity like 10W as compared to 20W are easily pumped through engine while 20W is heavy to pump. 10W and lower are smoother and fuel consumption is better with these. 5W even much better.
3 - From sticking point of view, oil should be of good quality to stick a layer on parts not just water, i.e. something quality of oil not grade.
4 - One role of oil is to allow or become a medium to transfer heat from parts.
5 - Again, it depends upon oil base stock mineral as they leave more wax/sludge on parts than low viscosity oils.
I think the quality of petromin would be good but should bring Customer needs in product line not to impose something.