It depends on what you are willing to buy, and what you want to pay.
Purchasing rules will vary depending upon what you have stashed.
Currently, I am probably ~100 qts or so, so I don't really need to buy any oil for some time.
Therefore, unless it is dirt cheap, I pass on it.
If you have a stash, you build it by buying when the oil is really cheap either on closeout or with MIR.
It is hard to pass up PP at $1.49 on closeout, or $1.00 after MIR, for example. You then have plenty of a very good Grp III product to use in anything.
At the moment, I have 22 qts of PP, probably more, so if I think the little Subaru might benefit from a run or two of it, for a little cleaning, I have it on hand, and it was really cheap.
I also have plenty of filters for it, since it uses the same filter as the Accords, unlike our Forester, and I bought a bunch of the appropriate Puro classics in '09 using the $10.00 off $25.00 coupons that were then available for AAP, and I also stocked up on P1s using the online order discount, as well as the 40% off online orders coupon codes that AAP has been offering recently.
You buy quanities you'll use over the next couple of years when you find deals, and you can then avoid buying at full retail when you need to do an oil change.
Just like laundry detergent or toilet paper.