What's different about a front differential?
 

Making sure I have good maintenance baselines on my newly-acquired E250 Bluetec 4Matic. No record of the rear diff fluid ever having been changed, so will do that ASAP.

Curiously, there is no recommended maintenance for the front diff. And the shop foreman at my local MB dealer claims that it is "non-serviceable" on my model.

Now it is clearly more difficult to replace the front diff fluid as there is a drain hole BUT NO FILL HOLE. You either have to remove the axle or employ this method:

https://www.youtube.com/watch?v=JM4yjivFn3w

But what I'm wondering is this: why does MB have a regular interval for the rear diff (40k miles) but assume/imply that the front never needs to be touched? Put differently, why wouldn't MB want to change the fluid since it's clearly something they're capable of doing and could charge the customer for?

This is something that seems to be common for front diffs, I owned a Ford Escape (Mazda MPV) for awhile and its front diff/transfer case is a grenade waiting to explode. It is supposedly "lubricated for life" as well.

It's the opposite with no drain hole and only a fill hole. You have to remove a bunch of shielding and then suck out the oil with an extractor from underneath. However in most cases it has turned to tar already and the bearings are toast. And it's expensive and absolutely terrible to extract from the car so we ended up scrapping the stupid thing rather than change it. It was rusting out anyways. Don't buy an Escape.

Front diffs do tend to be under less load as they are freewheeling in many cases and only engaged when 4WD is in use, but I don't see why this would actually result in less wear to the fluid as opposed to the gears. It's still being squeezed between the teeth with every wheel rotation.

I would recommend changing it if you can because odds are it is in just as bad of shape as the rear diff oil.

Front diffs on my trucks tend to get murky fluids because they don’t get as hot. If it’s truly sealed, it still breathes and can condense water in the interior.

I’d check again for a fill setup. There has to be a check port unless the drain hole is it.