The dashpot oil sold by Burlen Fuel Systems is SAE 20 oil. In the book, Paul states that normally 20W/50 engine oil is recommended (page 48). Why the difference (50 versus 20) from the manufacturer's recommendation?
This is a very interesting question that covers two areas. I will answer them separately.
The damper in the dashpot serves two purposes:
SAE 20 is a single grade oil which means it gets thinner as the temperature increases. Single grade oils were all that were available until multi-grade oils were introduced around the 1970s.
When you see an oil rated as 20W-50, the oil is a multi-grade which simply means that the oil falls into two viscosity grades, in this case 20W and 50. This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down.
A 20W-50 oil has the same viscosity as an SAE 20 oil at (I think) 30C. The only difference in the dashpots is that SAE 20 will get thinner than a multi-grade as the engine warms up. Nowadays with multi-grade oils being available relatively cheaply. I use my normal engine oil in the dashpots.