The offroad 4x4 has almost uselessly low performance - it cannot climb even the slightest incline. It seems as if the gearing for the first gear has somehow been set absurdly high.
In addition, the neutral position will sometimes propel the car forwards, and sometimes the reverse gear will also propel the car forwards, leaving you unable to reverse. This seems to occur after the server has been running for a while and the car has been used a lot.
Repair a car, try to drive it.
As you can plainly see in the videos, there is no 'check engine light' or smoke coming from the engine. Either way, if something is supposed to be wrong with the car, having a low level of oil or cooling water would not cause bad performance. It would cause smoke at first and then a breakdown.
The issue appears to be caused by exiting the car while it is in gear, the game then automatically throws it into neutral, but the car never works properly after doing this.
Further testing needed to verify..
As a temporary fix, you can spam Q to downshift so the hud reads the car as in Reverse. Once in reverse, continue mashing Q for a few seconds, then slowly shift into neutral and then first. This seems to be a sync issue between the hud and the car. Leaving the car while it is in any gear that isnt neutral will set the hud to show neutral regardless of what gear the car is in, and changing gears from that state will cause issues. spamming Q for 10 seconds before beginning a drive and then shifting from there will help!
To add further, there does seem to be an issue with vehicle performance on hills. The ADA 4x4 has ZERO testicles when it comes to any sort of hill climb, especially off road. Like OP said, it is like a car trying to climb a hill in fourth gear from a dead stop, the gear ratio for first seems absurdly high and this has prevented me from accessing my base with vehicles.