Turbo Lube oil bath hubs are clearly the best. Tie Down Engineering specs 50w engine oil min or up to 90w gear lube. Gear lube is not hypoid. It does not have high pressure additives for differential use.

50w oil and gear oil are not hard to find. Why wouldn't 40 w do? Had early oil bath system that speced. 30w and it worked perfectly for over 20 years.