I have a McCulloch strimmer which takes a 40:1 petrol/oil mix.
However, my mixing bottle (which takes 1 litre of petrol and has a
pointed top graduated in different radios) has become 'bloated'

This means that when you fill it to the 1L mark, there is actually
more than 1L in the bottle.

If I then top it up to the 40:1 with oil, I am getting a slightly lean
mix.

So, to counteract this, I have been adding a few extra drops of oil,
but the problem is that I don't know what ratio I am creating this
way.

I imagined that a slightly richer mix would be better than a lean one,
so I have made sure that I mix is on the 'rich' side if anything. I
figured that if it was too lean, then I stand the risk of seizing the
engine and doing real damage. But it is too rich, I would just have
too much oil, resulting in more smoke from the exhaust.

However, I asked my local lawnmower maintainer about this and was told
that it should be spot on 40:1.. To much or too little oil is just as
bad for the engine.

Is this really true? Is it a real problem running a 2-Stroke engine
with too much oil?

Even if too much oil is not ideal, is it so bad that you should throw
away a mix, if you added too much oil by accident?

Can anyone shed any light on this?