It depends. You’ll need to make your decision based on the way your business operates.
If oil changes are your main service, you need a large oil drain (20 gallons or more)—possibly several of them. If your specialty is in another area, you can get by with a smaller, 8- or 18-gallon oil drain.
Larger Oil Drains
You should invest in a large oil drain if changing oil is the main focus for your shop. A 25-gallon or 27-gallon oil drain would be ideal if you do a significant amount of oil changes on a typical day. You may even want to consider having multiple oil drains if oil changes are your primary service.
Smaller Oil Drains
You need to go with a small oil drain if your shop’s specialty is something besides oil changes (such as brakes and alignments). An 8-gallon or 18-gallon oil drain would be more suitable in this case. A small oil drain is a more affordable option for lower-volume repair shops, or even for DIYers at home.