My friend and I are a general repair shop that is out of the city limits a ways. We will work on pretty much anything that breaks. ATV's, snowmobiles, mowers, auto/light truck, medium/heavy truck, ag... You name it. We also deal in filter sales, tire sales and hydraulic hose making. To better illustrate what I do- my week has consisted of a transmission swap (which snowballed into several other repairs), a head gasket on a tractor (still working on it), repairing transmission cooler lines on a hacked together Focus, a Kawasaki Bayou 4 wheeler in need of a little TLC, oil change and leak repair on a Ford E-350, oil change and check over an S-10 Blazer, rear tires on a 2500HD Chevy and probably something I'm forgetting as well as all the phone calls and parts chasing it takes to get things moving along.
We buy our oil from several places. Local auto parts stores, farm stores or even larger distributors that we have accounts with. We buy based on brand and price. I prefer to sell a name brand of some kind over a "house brand".
Very few people make big money on oil- distributors, stores and us included. So it's hard to mark up and still be competitive/fair since we don't get squat for a discount. On our passenger car motor oil, we make decent money, but we watch the sales fliers like a hawk- and yes, even here where members have pointed out some killer sales. This allows us to sell a quality name brand product at a good price and still profit nicely. We buy in cases of quarts- because we don't don't really do the volume to justify drums.
Hydraulic oil and diesel oil are bought in 55gal drums, but we also use it for ourselves- as my friend farms and has several tractors to maintain. When billing, I'll just take our cost, divide by 55, then by 4 to get price/qt, then mark up 15%. Other lube and shop related goods like antifreeze get a 15% mark up. Repair parts vary in mark up, but the typical auto part gets 30% up to 100.00, then scaled back from there. So if I bought a gallon of antifreeze for 11.99, I'd sell for 14.11. A 39.99 auto part will be resold for 57.13. Auto dealer parts typically just get resold at list. Repairs are billed by the hour- no book time for me. For an oil change we get 3.89/qt of oil, unless it's not from regular stock- like Mpbil 1 0w-40, then it's price plus 15% and a 30% mark up on filters- but we get good pricing on main line, name brand filters so the customer still gets a more than fair price.
We don't do "specials" in the way you described. Again, not enough volume to profit from, and at 15.00 an oil change (plus materials) for a non four wheel drive, I'm hurting anyway. We do send out flyers from time to time on tire or filter specials- typically spring and fall to catch our farm customers.
Not sure if I answered your questions, but I hope this sheds a little light on the subject. Just keep in mind that we operate a bit different from a middle of town, auto centric repair/oil change shop.
As an aside- customers really make the shop. We've been blessed with an abundance of really good, down to earth customers and that makes doing business MUCH more enjoyable. In a way, I'm glad we aren't in the middle of a metropolitan area.