Used motor oil or fresh? Fresh, I wouldn't be concerned if you cleaned them out well first (just hot soapy water should suffice). Used motor oil can have some metals (hexavalent chromium, aluminum, etc. from wear in the engine) that you wouldn't want in soil for fruiting plants.
Not really sure it's an ideal planter in the first place though. Pretty small for trees, and unless you use a real high quality coating (i.e. a novalac epoxy, not paint) they'll rust pretty rapidly in contact with soil.
Metal oil drums are useful for many things, but I would not use them as planters. Too many problems.
Can they be cleaned? If they are from new motor oil, they can be made reasonably clean (but not certified organic clean). Drain, wipe, soap and hot water, maybe TSP. All the recovered oil has to be disposed of properly -- not poured down the drain. Used oil has a lot of combustion byproducts that are problematic, so cleaning is harder.
And then, they will rust out very quickly. It's not worth the money to paint them (and paint is not wise for food producing plants anyway). I guess you could put in a plastic liner.
Anyway, there are much more suitable choices for planters.
posted 1 month ago
Thanks for all your imput!
You guys have pretty much convinced me not to use them. The oil drums I've got had used motor oil in them.
I know they are small for trees, but for the yard space I have available, small trees would do fine.
Plus, as containers go, oil drums would be a pretty good quick fix, I thought. Hard to beat the cost factor (zero).
I think for fruit trees like yours the size is fine (I grow fruit in even smaller containers) but I would not use them for food trees either. Save them for your rocket heater or other projects (furniture?)!
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit