The transfer case cooler is an issue for me, I want to drain and refill my transfer case before 9 or 10k miles, not confident in doing that yet. The diffs, I've read many posts, one person spoke with an INEOS rep or mechanic and they said 22 mm for front diff, 32 mm for rear, as you write above. I'm not certain that any mechanic randomly selected will realize that they need to fill Grenadier diffs to those specs, they'll fill to the bottom of the fill hole. So is it really 22mm for front, and 32mm for rear?
I've noticed over the last few decades that all vehicles essentially have settled on the same ways to arrange the windshield wipers, lights, and other stuff around the steering column. Having diffs that require fluid levels to be specific mm's below the fill holes kind of breaks things in a sense, because most mechanics will do what they have always done, fill to the bottom of the fill hole.
I have just changed my transfer case oil (only). It's not difficult. You just need to keep an eye on how much oil drains out so you put it all back in again. This was the transfer case oil only so 1.8 litres out of 2.64 litres total oil capacity, or 68%.
I still have an open case with IA customer service for a document that covers a combined transfer case
and oil cooler drain and fill. In the interim I used the available documents to change the transfer case oil.
Transfer Case Oil Change
The recommended oil is Fuchs Titan Sintopoid 75W-90. 75W-85 is also acceptable. See the Ineos provided Quick Reference Guide (fluid) Specifications available from
here and
@Rok_Dr's Owner's Manual Supplement
here.
The transfer case oil capacity is 1.8 litres. The combined oil capacity of the transfer case and the oil cooler and plumbing is 2.64 litres.
There are two distinct documents visible in the workshop manual concerning the transfer case oil:
1. Transfer Box Oil (Check). This includes instructions to
fill-to-spill when adding oil to the transfer case.
2. Transfer Box Oil (Drain and Fill). (two docs appear with a different ID# but have the same content). These instructions are for a drain and refill using the same oil. It's the same process to do an oil change.
I have screen grabbed those procedures into the attached pdf.
To be certain I had not lost any oil from the cooler circuit, after I refilled the transfer case I temporarily reinstalled the filler plug then idled my vehicle back and forth on my level driveway a couple of times. This was to operate the oil pump in the transfer case (see below) to ensure the cooler circuit was full. I also operated the CDL and changed from high to low to high to ensure all parts of the transfer case were full of oil.
I then did a 2nd oil level check at the transfer case. It was still full and spilling. If the level had dropped I would have topped up and repeated the process until the oil level stabilised. If you did this job on a hoist you could do it with the wheels turning in 1st gear while clear of the ground. When the transfer case won't accept any more oil you know everything is full.
Notes:
1. I did the oil change cold. No particular reason other than it was already a hot day without working around hot oil as well.
2. To ensure I was only draining the transfer case I measured how much oil came out. I collected ~1.8 litres. When I replenished the transfer case I ensured that ~1.8 litres of new oil went in.
3. There was ~0.84 litres (2.64 - 1.8) of old oil still in the oil cooler and pipework (see below). The old oil looked clear and there was nil debris on the plugs so I left that oil in the system for now.
4. The drain and filler plugs are sealed with o-rings and torqued to 15Nm.
Oil Cooler and Pipes
There is no drain plug on the oil cooler and no service point in the pipework. There are separate documents in the workshop manual concerning Transfer Box Oil Pipes (Disconnect and Connect) and Transfer Box Oil Cooler (Remove for Access and Install). Those docs simply state to add additional oil when doing the after-maintenance oil level checks. I'll update this post if I get something from IA on how to do an oil change on the transfer case and oil cooler at the same time. I know how I would do it but I first want to see if there is an IA procedure.
Transfer Case Oil Pump
There is a gerotor style mechanical oil pump inside the transfer case. This pump is mounted in the rear cover of the transfer case and driven by the intermediate shaft. It circulates oil to and from the cooler whenever the vehicle is moving (the intermediate shaft must be turning). A Gerotor is a positive displacement pump so oil should not drain through it when it is not operating.
I'm going to try to hold my tongue on this thread as best I can. 
I've seen some crazy stuff in the mechanic world.
I am still waiting for access to the online manuals -- received email to request access on 3 October and submitted my information, have not heard back yet.
