Why do they paint the drain/inspection plugs on differentials?

I believe it is part of the manufacturing process. The guy that fills the reservoir with oil splashes on a little paint at the end of the process. This serves as a quick, easily observed, confirmation that the oil has been added.

Ed

Actually, it's probably the Jiffy-lube (or whoever) playing "Cover your ass". Apparently, not too long ago, there was a rash of "Check/change the oil in the diff, please" that was the prelude to a scam - The scam being that the oil would be changed/checked/whatever, and everything was good when it left the grease rack/pit, but then a short time later, the vehicle would either be brought back, or the information would come back to whoever did the work that the owner was pissed because they didn't do the job right - too much oil and blew out the seals, too litlle oil and tore up the ring and pinion, no service at all, but I got charged, there's something inside that doesn't belong, or whatever - and he's coming to collect payment for his vehicle being "ruined" by their "incompetence", when the failure was due to either his own bungling, or an outright attempt at fraud.

So, instead of just threading the plug back in and calling it good, they now slap a tamper-mark on it when the job is completed so that it can be told at a glance whether or not the plug has been removed. (in order to add or remove oil, or toss in a handful of BBs or sand, or whatever else might have "gone wrong" since the last time they worked on the vehicle.) Tamper-mark broken equals "It ain't *OUR* fault. Look! We put a seal on the plug when were were done doing our job, so somebody else had to have removed the plug and changed the conditions inside the diff from what they were when we last saw it. Who knows what some other idiot might have done to the poor thing? IT sure wasn't our fault, though!"

From what I was told, when the tamper-mark first started being placed, there was an amazing number of claims that were made that were tossed out immediately after the mark was revealed to have been messed with, and a whole lot of money saved by the oil-change/lube outfits because of it.

Marking the plug that way would also be an aid to preventing warranty claims on new vehicles - "We told you to have it serviced at 1500 miles, and you didn't. How do we know? Because before it went out the door of the showroom, we put a glob of paint on the plug so we'd know whether it had been opened or not. The same paint glob we put on when the odometer read "0000001.8" is still there today, untouched, with the odometer reading "0007839.2". That's four times the interval you were told it needed to be done at. Improper preventative maintenance. Warranty claim void. Have a nice day, and don't let the door hit you in the ass on your way out."