5KS Final Drive and ATF fluid check

["Dave C." <conner@cfm.ohio-state.edu>]

Bummer.
I wanted to be anal and check the fluid level in the final drive found in
cars with the automatic trans.  This is an 87 5KS.
It is nearly impossible to find the filler hole, but if you look in the
right place, it is barely visible from underneath, just aft of the drivers
side front axle and about level with same.  There is an engine mount bracket
blocking the view of it.
The bummer is I couldn't get the plug out and damaged it trying.
I used a box wrench to turn the hex key and found it very tight.   I forced
it and it looks like I split the side walls of the plug where the hex key
fits in.  I didn't think this was possible, I've used more force in the past
on these, but I guess this time the plug was protruding with the side walls
unsupported.
Has anyone else ever damaged one of these?  What will it take to get it out now?

Why was I being anal?  I'm worried about the oil slick on the ground and
wanted to be sure the final drive still has fluid in it. 
There is a slow steady trans fluid leak ever since I replaced the ATF trans
fluid, gaskets, etc with synthetic.  I removed the pan and reinstalled
everything thinking maybe the gasket was on wrong.  All seems in order.
Checked torque on bolts, etc.  Maybe the gasket doesn't get along with the
synthetic fluid?  Maybe the problem will go away if I put cheaper
non-synthetic ATF in?
bye,
Dave Conner
86 4KQ, 87 5KS, 89 100