Well you can,I use it one door hinges at home and I have used it on a car,it is also great if you have a slow retracting seatbelt just pull that all the way out and lightly spray the start and work in and out,be careful not to use too much as the seat belt will retract at a serious rate of knots.
Might be a part in the door thats faulty like the door strap . You could use oil on the hinge first but grease would be best .Put some rags under the hinge before you oil it and after wipe off the excess .
Yes may be BTB 500 and if you righ i would get it changed asap .If it fails it can damage the door or even the front wiong as the door swings out ..The bolt holding the one part of the check strap that is connected to the door elongates and woll snap off .This lets the door swing out to smash in to the front wing .Or even shut the broken part of door strap in the door then this will damage both door pillar and door .
I had a door check strap replaced on my SL at a main dealer a few years back - about £80 all-in IIRC. Probably a bit more now, but it's not a big/expensive job.