There is absolutely no way in any universe to mistake the expression that smacks into Optimus's face as anything other than vaguely horrified concern. He tries to keep his emotions like that under wraps around the Terrans to avoid making them uncomfortable, but he can't help this one slipping through!

"You mustn't wait until you are zombie before seeking out fuel. The healthiest systems are maintained by regular intakes at the correct levels!" He parrots, having had the lecture from his own medics many, many times. Any hypocrisy doesn't escape him, but he at least puts on a slight accent to try and dampen the impression she's getting a lecture. "Let's make sure it's at least every other day from now on, as you certainly don't want to get overcharged either."