The matter isn't "either"/"or", it's "both". Some of the fuel is subject partial oxidation during hot conditions before it's harmlessly evaporated and recycled through the combustion chambers, burned and expelled through the exhaust system. This partial oxidative state happens in the combustion chambers, too, as evidenced by coke buildup on valve faces, piston crowns, and ring lands. Those oxidation products in oil, given the chance, will clump to form insolubles and/or directly further react with the oil forming more oxidation products leading to the varnish and sludge buildup seen in poorly maintained engines. Detergent/dispersants go a long way towards neutralizing and holding those nasties in harmless suspension. The formation of these oxidation products is unavoidable with any type of driving, but urban crawl, hot weather, and/or high output operation (all defined as "severe service") accelerate the process. Fuel oxidative products react especially well with sulfated and polycyclic aromatic contaminants rich in double bonds - neither of which are present in Group IV base oils, and nearly so in Group III base oils. Group I base oils have the highest amounts of these contaminants. But, given sufficient time in use, even synthetic oils <i>will</i> eventually start down the reactive path, too.