Mech fluid is a substance that exists in Transformers, at least during the Beast Era. It seems to be analogous to blood in conversation and metaphor, but its actual function has not been expanded on, nor its link to any of the other fluids Transformers have been seen to have inside them over the years.
When Tarantulas prepared to drain Cheetor, he reminded the Predacon that he had no blood, just mech fluid. Tarantulas explained that he preferred the act to the actual nourishment. That's just gross, even for Tarantulas.
Rattrap once mentioned to Silverbolt that he knew of a place on Cybertron where "ya can get dirty mech fluid mixed with just a touch o' radium" which takes a Transformer's "head right off". Given the that the serving-bots at this establishment also have their torso-plates removed, the morality, safety and possibly legality of this practice is dubious at best. Rattrap also didn't want young impressionable minds listening in on the conversation.
During a rather heated wrestling match between Bumblebee and Cyrus "Colossus" Rhodes, a piece of Bumblebee's exostructure was knocked off, dripping with mech fluid. Prometheus Black picked up the fragment after the battle and took it to his laboratory, where he combined a sample of Bee's mech fluid with his steroid solution, creating a powerful acid capable of melting Autobot armor.
Not sure how that works, but okay...