I think you and your friend are both right. You should definitely avoid detergent engine oils. The closest ISO Viscoscity grade to SAE 10W is ISO46. This should work in your dozer.
The suggestion to go up to ISO68 may have been suggested because higher in-use viscosities will improve efficiency and productivity of the equipment. The typical drawback of switching to a higher visc grade is degraded performance at start-up and cold temperatures leading to possible cavitation.
To get both you need a multigrade hydraulic fluid that has improved low temp as well as high temp performance. I would avoid the 10W20 fluid because multigrade engine oils are not shear stable enough for hydraulic applications.
You should look for a shear-stable, hydraulic fluid formulation with a viscosity index of at least 150. Some manufacturers sell products labeled MEHF (Maximum Efficiency Hydraulic Fluid) that meet the performance level you are looking for.
Hope the helps,
-Doc