In the MCU (I have never read the Guardians in comics) Drax seems to never consciously lie. Any time he speaks it seems as though he tells the 'truth' as he understands it and of course is completely literal when doing so. I do not remember him ever embellishing any truths to seem more grandiose than he actually is; others do that for him.
During Infinity War there is a scene where Gamora is speaking to Quill and she gets him to swear that he will kill her if Thanos captures her because she knows something he doesn't. As this scene starts the camera moves forward, about 10-15 feet, into the area where the two will talk. It moves between two massive cylinder things; centered between and in front of which is a small table. There is nothing in between these cylinders as the camera moves forward...
As Gamora and Quill begin to kiss we hear off-screen a crunching noise and the camera reveals Drax loudly crunching on some nuts. He is standing in between the two cylinders, nearly leaning on the right cylinder. This sparks the dialogue:
Quill: Dude, how long have you been standing there?
Drax: An hour.
Quill: An hour?
Gamora: Are you serious?
Drax: I've mastered the ability of standing so incredibly still that I become invisible to the eye. Watch.
Quill: You're eating a zarg-nut.
Drax: But my movement is so slow that it is imperceptible.
Gamora: Mmm, no.
Drax: I'm sure I'm invisible.
Mantis: Hi, Drax.
Drax was not visible standing there when we saw the spot previously and this dialogue obviously lasts less than an hour which sparks my question(s)... Was Drax standing there for an hour, or was he lying? If he was lying, has he done so knowingly previously? Or as I suspect, has he in fact mastered the ability to stand so still that he becomes invisible and he simply lost his concentration because he was hungry for a zarg-nut?