Do you need this on a 2006 2500hd if you are changing the front axle fluid to 75w-90 synthic that white cap, and new o-ring that's resistant to the synthetic oil?

If you haven't checked it already it may have it. I looked at my 04 before I did mine and it was installed already from the factory.

You should have it on an 06.

I've got 30k miles on mine with synth in the front and the black vent. No problems so far. Others have said the same, so I wouldn't be too worried about it either way.

Ok thanks alot guys for the info