Ive had both the 2012 and the new type oil tank. The new one is much easier to check the oil level as it has two planes in the tank to look at rather than a dipstick which is hard to use if the oil has been sloshing around.

Changing the rubber material is the point yes. The problem is that viton is the rubber best suited to resisting the dissolving effects of pressure heat and oil. The rubber needs to withstand over 150 deg c. Looking at data tables this rules out nitrile et al.

Viton is the correct spec but i find viton is quite brittle to work with, breaking up into tiny little chunks when you cut it. The same happens when the cap closes on the tube and cuts it.
hence why I believe the interface surface areas need to be increased.

Its actually quite a complex problem...