I just talked to some garages, as long as you have a good way of picking up - they are mostly worried about spills. I have a gear pump and 1000L tote on a trailer, they call me when they are full and away I go. You only really need 1 maybe 2 shops. Dropping off the odd case of beer helps
takes me 15-30 min to pump 1000/L
I also provide a tote/barrels, 900L home heating tanks etc. for them to store in if needed. My best interest that they have large storages so I don't make multiple small trips.
I don't worry about contamination, that is what settling & filtering are about. Coolant settles out in less than a day. I have 2 auto shops and one farm shop. I get hydraulic, motor, tdh(lots) synthetic you name it.
Worse is hydraulic with water that has been run - turns beige and doesn't separate but does separate from the other oil in the bulk tank. Is useless - hardly even burns in the burn barrel.