I'm not 100% sure. Recently Scawen implemented the "CPW" system which checks for mods, and I'm pretty sure this checks for VOB mods. However if your VOB mod has a "check value" that is the same as the normal car then it gets past the check, although maybe this means that it doesn't affect the collision physics. I'm no modder so I'm not sure
The cars you've seen on youtube are known as "VOB mods" where the VOB file of the car (which details what it looks like) has been edited to make them look like these other cars - they are the same as the standard LFS cars underneath. It's not allowed to discuss VOB mods on the forum as they're modified content but they are freely downloadable from a quick google search. Be careful that any mods you do use don't affect the collision detection system, which means you'll crash even though no-one has hit you.
Team Name: Need Dinner Racing Number of Cars Entered: 1 Car Numbers Requested: 69 Team Manager Real Name, LFSWorld UserName: Jack Atkinson, J@tko Drivers: Jack Atkinson, J@tko, England
ZOMG GUYZ - WE NEED TO REMOVE CUSTOM VIEW TOO COS THAT MAKES YOU EVEN FASTAR1111!!!
[slightly more serious]This pretty much explains why custom view is much worse than forces - you see everything you do in forces without the forces [/serious]
Why didn't anybody explain to S.E.T.H. that driving in F-mode does NOT reduice the weight of the car to only four wheels, a driver and a steering wheel and therefore is NOT faster than the "normal" mode and will NOT give you faster times?