When you *first start it*, there is fuel *accumulating* on the cylinder
walls, fuel that either condenses due to contact with the cold surfaces
or fuel that was not evaporated to begin with. After all, a cold start
mixture is typically as low as 9:1 but I've seen them as low (rich) as
5:1, much richer than a warm engine requires (14.7:1. Even an engine
under hard acceleration only requires richening to about 12:1 and,
because the engine is fully warmed up, this fuel will *mostly* vapourise
and be burnt.
The point to be made here is that *liquid petrol* does not burn. That
means, when running a cold engine with a rich mixture, a considerable
quantity of fuel remains in the cylinders in liquid form. That liquid
fuel is able to pass by the rings due to increased clearances when the
engine is cold. That fuel mixes with and dilutes the engine oil. If you
then go for a decent long run at highway speeds, that accumulated fuel
in the sump will then evaporate aided and abetted by the PCV system.
Particles, such as those found in the fuel additives may not evaporate
and will remain.
A good example is an injector cleaner I used in my previous Corolla. The
injector cleaner worked perfectly well freeing up my sticking injector.
That injector cleaner was added to the tank of fuel, like your Nulon
additive, but the instructions on the bottle clearly stated that I must
change the engine oil immediately after use, ie. once the tank of fuel
was emptied. This I did, and the reason was that the additives will end
up in the sump mixed with the engine oil and may form harmful compounds.
The additive is, after all, a pretty efficient *solvent*. It needs to be
since it needs to remove gum and varnish from fuel system internals. I
would suspect that the additive in the Nulon would also have some
solvents as part of its makeup. In general, fuels these days are pretty
good but 91 tends to be only passable and will lead to gum and varnish
buildup. you would get more benefit from occasionally running a tank of
98 octane fuel through your car since that fuel has absolutely no added
ethanol and much more effective fuel system cleaning agents, agents that
will not cause issues if they end up in the sump mixed with the engine oil.
HTH