Paul Bowden wrote:
This is getting stupid: apologies! I MEANT AIR FILTER (of course)

After running my grandfathers old ATCO for many years with home made air
filters scavenged from the car after servicing, I would have thought that
oil was the last thing you would want anywhere near an air filter. It
strikes me that it would immediately clog with dust if air could get through
it at all.
Try starting the mower with the air filter removed, if successful, then get
a clean replacement.

Toby.