Engine Oils

I was going to put this into the Daytime Running Lights thread but decided that is was different enough to have its own thread.
I appreciate that engine oil technology has moved on significantly compared to my youth where the range of choice was to use either (green) Duckhams Q20/50 of Castrol GTX, however...
Today we have had a 4 year old 'ordinary' Vauxhall Corsa in for a service and have discovered that there is no after-market engine oil available. The oil requires can (currently) only be made by Total and it is only available from the dealers at the bargain price of £106.60 + VAT for 4 litres!
I know that a lot of manufacturers have been suffering with degradation of timing belts since some bright spark decided that it would be OK to run them inside the engine, and therefore surrounded by oil, so I am presuming that this magic oil has some special additives to prevent the degradation but at over £25.00 a litre (plus VAT) will I'm encourage people to use cheaper 'normal' engine oil which could lead to a catastrophic (or to put it another way - expensive) engine failure due to what most old school engineers would class as a dumb design to run a (basically) rubber belt in engine oil.
(Ford have experience belt degradation problems with their 1 litre EcoBoost engine and have recently re-designed it removing the belt in oil timing belt, and replaced this with an old-fashioned timing chain, which seems like a good idea. However the oil pump was also (rubber) belt driven and this is apparently still the case following the redesign!)