As Exxon-Mobil holds the 50% of synthetic market share, they can obviously leverage the price via supplying vast amounts of their product to the market.
In terms of economics, the very low price could probably result from Mobil over-supplying the market, relative to the level of demand, which would greatly decrease the price customers (i.e. wholesalers/wharehouses which distribute to the likes of AutoZone, Walmart etc) would be willing to be pay for the product.
This is why some boutique oils, like Redline or Amsoil are often so costly; because there is a lower level of supply, which possibly isn't meeting the demand, which would therefore drive the prices upwards (as people are willing to pay more to get the limited quantity of "good stuff").
Of course, this is assuming that Mobil is simply flooding the market. It could very well be that they can afford to run such a low price (economies of scale), as they probably sell a lot of synthetic base-stocks to other blenders/manufacturers, and use their profit from that to offset the cost of Mobil 1 products.
In the end, if Mobil are flooding the market, it is an unsustainable position to maintain, as they are operating in dis-equilibrium and possibly loosing money as a result. This will eventually force the price to increase again overtime.
So in the case of Mobil, it'll be a case of "watch this space" over the next few years, and see what they choose to do.
