In the early stages of Megatron’s life, keeping clean Just Wasn’t A Thing. No matter what you did, you ended up caked with dirt and grime with fistfuls of pebbles rattling around in your vents at any given time. The most the miners ever did was scrape off the thickest of the muck and scrub out their internals at the end of their shifts.
Wartime hygiene was spottier. Sometimes Megatron was able to keep up with bathing regularly every few cycles, and sometimes shit would hit the fan and he’d be too busy and obsessed to even think about a shower for more than a millisecond.
Nowadays, he revels in shower time. Hot liquid eases the joints and the mind like nothing else, and sometimes he’ll stand under the faucet for two or three hours just thinking and letting the solvent patter on his armor.