Looking at the internet, I discovered this link, this evening, although I cannot recall the source; it might even have been from the BBS.

https://www.pressreader.com/uk/airgu...82321090250316

The gist of it reveals that changing from a leather seal (not sure if this is interchangeable with the word 'washer', ie one and the same thing) to a synthetic seal would create a need to reduce in diameter the size of the transfer port - quite a technical operation. Moreover, airgun manufacturers expected their products to diesel and factored in a degree of this to create the necessary velocity. This rise in the velocity from the use of oil was, I suppose, obvious given that instruction manuals requested the insert of so many drops every 500 pellets. This set me thinking; given that neatsfoot oil compound does not diesel, or at least it does so at higher temperatures, I wondered if its use would defeat the object of using an oil. I wonder what technical experts, here, think?

In any event, swapping a leather seal/washer for a synthetic variety might not be such a good idea unless the transfer port is modified. This sounds like another exam question, were it to be followed by the word 'discuss'!