Shell DIALA is dryer than Tellus and can take higher contamination before it looses it isolation. I believe you are referring to tellus 22 S2 or S3? DIALA is, according to info I receive from a large ROV company that are using both types of oil is that DIALA is gentler on the insulation on the cables. And rubber/plastic in general.
The compensator is self made and have a volume of 100ml.
Not sure I understand the question about the tether?
To answer you last question, whats the plan: The plan is, like I stated in my post, to install 2 off PDU`s in the ROV. The size of these are way to big for the 4" enclosure. Thats way I am going with the compensated approach. A 1atm bottle and encaps that size will be quite expensive in comparison.