Lubricant

Lubricant is a darkish black fluid contained within the body of a Transformer that is (presumably) used to make the mechanical joints of a transformer move more smoothly.

It appears to be offensive to transformers to "leak lubricant" on them, as implied by Ironhide's near willingness to turn Sam's dog Mojo into a smouldering crater just by peeing on him. Although it is offensive to lubricate transformers, it is not seen as offensive (to the transformers) to leak lubricant on humans; especially if said human kidnapped and insulted (among other things) the owner of the only known way to locate the lifeforce of the entire transformer race.