A weight (wt) is a range of viscosity not a specific viscosity. Here's a couple a charts, that don't completely agree by the way, that show how various wts compare with actual viscosity.
http://www.bgprod.com/blendr/pdfs/viscosityChart.pdf
http://www.powerstrokenation.com/for...ead.php?t=1503
When it comes to selecting an oil, decide on the range and pick one from the chart. The chart is sorted by viscosity at 40 C to make fork oil selection easy. Note that there are some shock oils in the list that don't make for good fork oils. BelRay HVIs are an example (they don't contain anti-foaming agents).