Having been QUALIFIED in motor vehicle repair I would say my "speculation" is educated and therefore not classed as speculation. Its common sense that the very life blood of the engine, the liquid that cools the parts, lubricates the parts, traps the piston blow past combustion products in suspension, transports the metal fragments to the filter and keeps the parts inside clean would operate better when new. Would you keep the oil in your fryer at home for 6 months? no because it degrades with temperature and particulates in suspension. It will still fry your chips but it will taste disgusting. Because something just works it doesn't mean its the most perfect solution, changing the oil in your engine more regularly than you should will bring more benefits than leaving it in longer, it doesn't take an educated man to work that out. Just one with common sense and an understanding of the purpose of engine oil.