Don't know how to get an original out as I have only worked on already ripped apart ones.
RS calls it: "Heat Sink Grease", p/n 276-1372, $1.99. Your stuff will work just fine. In fact, my probe is just hanging in space behind the bumper.
My 356C is currently buried behind and under all the winter crap in the garage and is utterly filthy. I'm very ashamed as it is a very nice car.
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet
'86 560SL 124K Miles
'94 320E Wagon, 74K Miles