Modern engines use lower viscosity oils because they are built to tighter tolerances and require a slightly different kind of protection than older engines, which need a better 'cling' effect from the oil to ensure adequate lubrication. The other thing to bear in mind that as well as being a relatively old engine, the 1.3-litre unit in your Polo is not a performance engine either, so is unlikely to be worked hard and require good high temperature performance from the oil. A fully synthetic oil would be fine to use in your car but is more expensive, and a semi-synthetic oil would provide as much protection as you could realistically need. We would suggest using either a 5W40 or 10W40, affording you some extra protection in winter use but also sufficient lubrication in normal use. Going for a semi-synthetic oil would also mean you can change the oil more frequently for less cost, which would be a sensible step to keep it running for many years to come.