Through bing translate the fuel etc. is only visivble from a certain camera angle and range. No mention of being able to turn it off completely however. Source:
https://vk.com/spintires?w=wall-33295160_141111

Hi, I just talked with the devs about this feature.

• Fuel swaying in tanks doesn't make the truck sway, although it might arrive in a future update. We'll keep you updated about that.

• The amount of fuel in cistern does affect the truck weight.

• It's currently not possible to switch xray feature (showing transparent fluids) off for the vehicles we have in the game, but it could be added if the community requests it. I already mentioned the fact that it was a frequent request on the forum.

• Modders can choose whether they include xray effects in their trucks.

Have a nice day!

Thank you.