I think I have finally cracked this conundrum but would appreciate readers' views.
I was aware that leather washers required occasional oil, via the transfer port. With my Original 45, the amount is a couple of drops every 1500 shots - very little. However, my Webley Mk3 seems to be thirstier. I have found that the power drops, perhaps by 20%, without regular lubrication. I know this because I have inspected pellets, fired against a flat steel target holder (I do not have a chronograph).
It was therefore a question of knowing when to lubricate. I have now reached the view that this needs to be done every 200 pellets, approx. At that point, I fill the loading tap to 50% (I use Weboil or neatsfoot compound), work the cocking lever and then allow it to soak in overnight in the upright position. I then shoot half a dozen pellets to clear away the excess and then the power returns to what I believe is probably a tad over 10ft/lbs (I can gauge this from the flattened pellets).
With the Original 45, there would be a loud crack with anything more than a drop or two, with serious dieseling as the oil exploded under pressure. That is not the case with my Mk3; the noise is more or less unaltered.
I am aware these rifles were made to function with oil; perhaps a degree of minor dieseling is a factor in the power.
Quite possibly, the size of the leather washer is also a factor. Perhaps, when it dries, the washer shrinks, thereby reducing the power.
(My Mk3 is a 1973 model; the tap works well and there is no leakage. On a good day, I can just about manage 3/4in groups at 25yds).
I should very much appreciate comments on the above. Is my situation typical? Am I over-oiling? Could it be the mainspring needs replacing if I am relying too much on oil for performance? It seems to me, however, that I have just about reached a sweet spot and that once the rifle ceases to smoke, new oil is required - ie every 200 shorts, approx. Does this sound about right or are there other things at work, here?
It seems the older springers have quite a number of factors to consider, to reach optimum performance. This, I find, rather adds to the satisfaction of firing them.