You might want to run some Dino oil through there for a couple of more changes to wash out all the casting material that may be in there. I switched around 10K to Mobil Delvac 1 reportedly to be "better" all around Syn. Oil (I am sure there will be a rise of alternative oil suggestions for that comment), but I believe it so there ya go!
You can change now to SYN if you want to, but you need a couple of quick changes (maybe every 3K) to wash it out good and using SYN for that is a waste of cash.
More important is changing the rear Diff fluid quickly. Some say as soon as 500 miles, but I went about 7K on miner before I was shown the errors of my way, and changed it.
You also want to change the Allison tranny spin on filter within the first 5K as well.
Do a search around here and you should have lots of explanations on why we all over maintain our trucks LOL
Enjoy the truck.