Gravity filter for three weeks 50 gallons at a time, have a drain on each drum about 6 inches above the bottom of the tank. When draining, filter through a series of different micron filters. Some guys like to centrifuge, but if you know what quality oil your getting from work, you don't really have to. Being that your at the shop your getting the oil from, you've already killed off most of the biggest scares. Don't let idiots but crap oil or mixed oil into your drums and you'll be fine.
Also I wouldn't go over a 60%(diesel) to 40%(oil) mix unless you know its low mile oil