D3 Transmission Issue

I recently bought an early '70's model D3 (79U) with a power shift transmission, 3 forward and 1 reverse. It's a nice little low hour machine. One of the issues I have is that when you start it up it won't move right away when you put it in gear. If you let it idle for 5-10 minutes it will run just fine, all day if you want to. You can turn it off, and when you go to crank it back up (in the same day) it will work just fine. It's not a deal breaker since so far it always works all day if I want to run it that long, it would just be nice to know what's going on. Any ideas? It does appear to have enough good fluid.



Also, does anyone know what kind of hydraulic fluid these machines use?