Bit the bullet today and bought 8 quarts of Royal Purple gear oil. I changed out the fluid in my diffs, 6 speed, and transfer case. The oil in the rear diff was nasty, stunk and had quite a bit of crap on the drain plug. Everything else looked great. My truck has 15K on it. If you tow which I do a lot it recommends changing it at 15K not 30K. It's a very simple task to do. I'll report back with any improvements in power or mileage. I'm going to switch the engine over this weekend too.