100 and 80 are quite easy to explain:
At a certain temperature, a given quantity of the oil will flow through a predetermined diameter hole in a metal plate in 100 seconds or 80 secs respectively!
So the 80 is less viscous, ie more runny than the 100 and is often used in winter time.
Apologies for the vagueness but it is a long time since I had to know the exact temp, quantity and hole bore!
The 20-50 is a multigrade but these appeared long after my time!!