Highest RPM you've seen

Posing this question to see just how high someone may be turning the Dmax. Reason being I got caught a little off guard decending a 9% grade into Death Valley California about a month or so back. The rpm's hit 4500 and I would stand on the brakes for about 10 seconds and release them for 10 seconds this would only drop the speed by 10 mph and after 10 seconds of no braking with my foot the truck and 5er would pick back up the 10 mph I had managed to scrubb off. I had to continue that process for I think 6 miles of grade, it was a little bit disturbing since I knew the Allison would automatically upshift at 4800 to save the engine. It spent what seemed to be 15 minutes of turning between 3800 and 4500 rmps. That evening I checked the oil and it had blown out 1/2 quart of oil. It has never used oil before and has not used any in the next 8000 miles of towing. Do you suppose I hurt anything in the engine by turning it that high for so long a time, or is that just nothing to worry about?

Oil usage aside, you can't hurt the engine by over-revving. The PCM and TCM won't let you do it. On compression (no fuel), it will run up to the top of the tach, and upshift at 4800 as you found out. This is normal within design, and I've not seen or heard of anything negative because of it. I've done this many times with mine (on that grade a few times), and know of many others who have done the same. At that RPM for an extended period, a bit of oil usage can be expected. That's a lot of air moving in/out of the crankcase, and the oil vapor will be carried away. If you plan on visiting this condition often, a vapor capture system may be a good idea (as well as an exhaust brake). Otherwise, gear down earlier and enjoy the scenery.

I don't plan on turning it that high ever, just got caught napping at the wrong time. I suspected that what you said about moving so much air was what was going on. It hasn't missed a beat all the way up to Prudhoe Bay Alaska back into Montanna, over to Mount Rushmore in SD and then back thru Wyoming, Colorado, New Mexico and back to Texas, still no oil used, and that's towing a max loaded 5er and truck at approx, 25,000 lbs. Yeah I know, that's pushing it, but don't ever let anyone tell you Chevy Duramax is a weak truck. With the Kennedy boost valve all the way in and The Edge set on number 2 the memory gauge shows a max boost press. of 27 and 1457 degrees F of exhaust gas temp, so I think it is still quite a healthy engine. 14,138 miles of towing and it seems to run better now than it every did. If only it weren't so hot and dry in Texas right now. I shoulda stayed in Alaska longer.