Some are much better than others.
Magnatec is a good example of an overpriced oil. It's not a genuine synthetic oil, it's a modified mineral oil, legally sold as a synthetic. A few years ago there was a legal case in the US where two oil companies went to court to dispute the naming of synthetic oils. One company believed that as some oils were so far modified from what they were when they came out of the ground, they should be sold as synthetic. The other company was one that only made genuine lab-made synthetics, so they wanted the modified mineral oils not to be sold as synthetic. Eventually the first company won and now all of the cheaper 'synthetic' oils for sale are actually modified mineral oils. Even a lot of the higher priced synthetic oils have a mineral oil content, but it can be good as it makes the oil more stable.
With Magnatec you are getting the basic level that can be sold as a synthetic, I'd describe it as a semi-synthetic if anyone asked me. Fuchs XTR is a very similar oil, but Fuchs didn't sponsor the World Cup etc, so works out as better value.
http://www.opieoils.co.uk/p-991-fuc...rmance-fuel-economy-synthetic-engine-oil.aspx
If you look at the label on the can, Castrol Magnatec is a 5w-30 synthetic oil that does amazing things, and it's only £24 for 4L. It doesn't really make sense that the Redline 5w-30 and Motul 300V 5w-30 are over £40 for 4L, when they don't meet the manufacturer specs and don't tell you how amazing they are on the can. The thing with the Motul and Redline is that they are both a mix of PAO and ester synthetic basestocks that are absolute top end oils. They give far better protection that more basic oils, are more stable at higher temps and last longer.
At the other end of the scale are the real bog standard oils, things that are £10 for 4L. There are always rumours that their oils are made by the big companies like Castrol and Shell, but the truth is, that really doesn't matter. If a big company makes an oil to a low budget, basic spec, that oil won't be very good. It's not just one of the bigger companies good oils with another label stuck on it, they couldn't afford to do that. To make the basic oils, a more basic additive package and basestock is used, meaning it's not as good a lubricant as it could be and it doesn't last as long.
Cheers
Tim