w stands for weight. so 15w is thicker than 10w ect. 15w is better for cars that have covered some miles.
Actually the W doesn't stand for weight. It stands for Winter and indicates that the oil has had it's viscosity (thickness if you like) tested at low temperatures. The 0, 5, 10, or 15 is then a viscosity index or 'grade' based on that low temperature test. A 5W30 will be thicker at 0C than a 5W30, but they will have roughly the same viscosity at 100C.
The 30 in 15W30 indicates the viscosity at high temperature (100C) and as mentioned will be more or less the same whether it's a 0W30, 15W30 or something in between.
A 15W oil will be thicker at low temperature than a 5W or 10W. This is not necessarily good for an old engine, which arguably requires cold oil to flow just as easily as a new engine at start-up. If you are talking about an engine from the 1930's then it might be a different matter, but any VW watercooled engine (i.e. from the 70's onwards) will be fine with a 5W or 10W oil. I'm not saying don't use a 15W if you are happily using one.
As for using an 8 year old oil, you should consider that the additive packages deteriorate with time, through oxidation and other chemical changes, as well as separation. Sediment and waxes can form, the viscosity index of the oil will change and then there is the risk of moisture contamination (even in a sealed container). The oil may look alright, but it probably isn't. When your engine gets hot, or when it is starting from cold it won't provide the protection of new oil. It certainly won't last as long.
Wayne is right - bin it.
