The locking gas cap on my new to me DT250B that is non operational, won't unlock.
I've been putting Kroil in the lock every day for a week while also using the key to wiggle it some trying to break loose whatever is going on.
The key numbers do match the lock.
I really can't visualize whats going on in the lock mechanism.
I actually got the key to go a little further in the lock kind of by accident. I thought they key was fully in the lock but suddenly it went in another 1/8"ish futher but I don't really know what is right. 
Is there an preferred method of ham fisting the lock into oblivion in order for me to unscrew the cap??.
Will a MX gas cap fit an enduro??
I'm kind of scared of what moonscape enviroment I will find when I look inside.
The petcock is also locked up but I haven't messed with it yet but thinking maybe I should remove it and fill the tank with vineger.
Any knowlege on this would be great.

Not sure whether or not yours is the same, but some dodgy photos of mine below. The key goes in about 15mm.

A good locksmith should be able to unlock it for you.

So I’ve a 250a and had a problem too. I pushed down then wiggled the key.. that worked for me. I have the style the cap is on a hinge and won’t separate from the tank…. A bit different fro Gr8Al’s

If you can eventually get the cap off, a 68-71 Yamaha Enduro cap will fit...  no lock.