I'm an engineer and work on heavy equipment and hydraulic system designs. Along with heavy equipment engineers, I've worked with mechanical engineers who were part of the Apollo space program and something that I've learned from working in these areas is to condition (filter and cool) the oil/fluid you have and not try to add more if possible. When designing systems, I don't always have the luxury of a lot of space, so am often forced to become more creative in cooling and filtering the fluid.
I decided to apply the same logic to a truck I owned and rather than increase the oil capacity, I researched heavy duty coolers and was able to keep my stock transmission pan temperature under 175F all the time. The cooler was made by Tru-Cool and I did the installation for under $100. Prior to the installation my oil pan temp was always 210-225F or more which would have guaranteed an early failure. When I sold the truck at 260,000 miles, the original transmission had not failed and was still working well.