web wisdom: Is there any need to use expensive oil?

philehidiot: There's seven quid oil and 14 quid oil. Same viscosity and same volume. For a bog standard road bike is there any point in treating your bike to the posh stuff or, for the average road rider, is it all pointless?

MikeRotherham: Check your manual, it will tell you whether your engine needs the cheap mineral oil or the more expensive semi-synthetic. The semi-synthetic has better long lasting lubrication and detergent properties than standard mineral oil so tends to protect better. 

Philehidiot: It's not so much the semi/fully synthetic thing. My bike uses fully synthetic oils - one is cheap(ish) and one stupidly expensive. It doesn't specify for track use, so is there a point in using them for road riders?

Bbstrikesagain: An interesting area... There are many oils sold by big brands as "fully synthetic" which aren't. Thanks to a stupid US law case the makers get away with selling highly refined oils with similar properties to genuinely synthesised oils as "fully synthetic" even though they don't contain any truly synthesised base oils. Ditto "semi-synthetic", it may contain absolutely no synthesised base whatsoever.
True "fully synthetic" oils are composed mostly of a synthesised base which is most impressive looking stuff, as clear as water, looks like glycerine or such, hugely pure, incredibly durable, and unlikely to tar anything up ever, but with an additive pack appropriate to the usage (adhesion and friction modifiers etc). The additives are usually in a mineral base, so even the best "fully synthetic" oils contain some mineral oils, and end up with a familiar yellow colour.
True fully synthetic oils prevent proper running in of new engines and fail to maintain seal swell in old engines, so only use where and when recommended. Could the price difference be the difference between a true synthetic and a psuedo synthetic, or just branding hype? Any oils that meet the manufacturer's recommended specifications should be fine – I doubt anyone needs to spend mega-bucks.