It's a tricky marketing landscape to navigate, but some of it is actual product quality differences, whether it's cost of materials or care in production, or a combination of both.
The one I find most interesting is the "middle" product placement, and the "premium" product (maybe "penultimate" is a better word), when they are solely related to consumer psychology and no significant quality difference.
The "middle" strategy is where you offer a low priced product, knowing that some consumers will always reject that one and choose the next higher tier, which is a high profit example (almost no difference in reality, but "placed" higher, sometimes by under-qualifying the lowest priced one. The low priced example carries less profit, but you are not trying to sell that one; it's just there on the shelf to sell the next higher tier.
By way of a non-oil example (because sometimes that helps see how it works with oil) let's say you have two audio amplifiers, each capable of 50 watts output at 0.05% THD. The lower priced unit will be specified at 0.1% THD, the next one at 0.05% THD. Neither is a lie, but there is a perceived difference that consumers pay more for. This is widely used, and it doesn't take much to see how one might do the same with oil.
Then there is the penultimate product, which is not really different from the middle tier (or maybe one higher) but is packaged nicer and is specified highest (tighter) even though it's not really any different. A certain percentage of consumers will always buy that tier, no matter what.
Another non-oil example comes from an anecdote, shopping with a friend for, of all things, underwear and socks, for a job we were going to do together out of town. I (the cheapskate) was driving, so we went to my store first, a discount department store. Checked out some brands and prices, and then without buying, we went to a specialty "work wear" store in the same mall, my buddy's choice of retailer.
Same brand, same exact item (it was the underwear, the socks were different) but at a higher price. I suggested going back to the original store and buying some. He refused, insisted on buying the higher priced items, because his consumer psychology just couldn't accept the cheaper priced items were the same. I'll leave it to the reader of this to guess what I did.
Anyway, from the above, you have four neat product grades, with two actual quality differences. A brand can expand or tighten the product lines as they see fit; it does cost more to have more tiers in the chain, so usually only well established companies can run four or more tiers of basically the same product. If the company is actually a quality manufacturer, even the least expensive tier is probably perfectly adequate for most applications, while there is still room for an actual premium tier for specific, heavy duty etc products.
It also explains why a value-priced marketed reseller (let's use Wal-Mart) still carries higher priced (and higher profit) items, say a store-brand jar of pickles alongside the national brand item, or why there are fifteen blenders from three brands in the kitchen isle.