I recently had trouble getting the plug wires off my '91 300E and almost had to buy the special pliers. Once I did get them off, I sprayed the inside of the boots with HD silicon and now they come off with no trouble. I've also used it to ease taking the air cleaner off the throttle body, lubricating the throttle linkage, easing the removal of plug wires from the distributor and on other rubber seals in the car.

I highly recommend getting a can of the stuff for both car maintenance as well as other uses like lubricating parts of the garage door and opener.

What your mechanic was referring to was getting any silicone into the intake can screw up (poison) your O2 sensor. This includes any silicone from excessive RTV on the intake and silicate from coolant. Otherwise, spray away.

Tip: I keep a tube of silicone (synonymous with "dielectric grease") in my toolbox. It's a must to coat the inside of ignition wire boots where they contact the spark plug ceramic.....a must for the sliding elements of brake calipers, and useful for all electrical connections to exclude corrosive environment.

__________________
95 E320 Cabriolet, 159K