Oil and petrol are like most other things, the more you pay the more you get, with good petrol you get cleaners, anti-knock additives and upper cylinder lubricants, (with supermarket fuels you get very few additives but sometimes get unexpected bonus ones like silicone which will zap your oxygen sensors). Injector cleaner additives do work, as I have experienced engine smoothness and petrol consumption improve after using.
With expensive oil, as well as being more refined (all that nasty tar and ash taken out) they also have more additives to clean and neutralise acids, and they also have anti-wear additives like MoS2 (molybdenum disulphide), which are chemically attracted to metal surfaces and create a barrier to stop metal-to-metal contact. For about £5 you can add molyslip to your oil, and the good news is that it stays in there even when you change the oil because it is stuck to the metal surfaces.
Check out the following link (this is about motorcycle oils which are normally far more stressed than the average car oil, but the basics still apply). Note the comments about not using oil with MoS2 if your bike has a ‘wet’ clutch (plates immersed in oil) because the additive reduces friction so much that it will stop you clutch from gripping.
http://motorcycleinfo.calsci.com/Oils1.html
