I'd have no concern about the buckets but I'd worry about ergonomics at some point.
The specific gravity of motor oil is about 0.9, so a 5-gallon bucket weighs roughly 42 pounds, bucket included. A standard 5-gallon bucket is about 15 inches tall.
Stack them 4 high and you're at about 5 foot tall.
Once you start lifting, moving, or twisting 40+ pounds above you shoulders, things can get interesting real quick.