defo skim for me. it's not a huge difference.
i find the "technique" important - regular stirring. this seems to stop the oil from rising (which you don't want to skim off for sure) whilst giving a long enough window ~20 mins to get all the scum out in skim form.
i get something like 300ml from a ~ 4L base.
Sometimes a bit confusing to follow Jerry
Is that 300ml of scum or oil from a 4L base?

Hi RubyDoo
It's probably down to knowing what scum is that your looking for on the final simmer.
If you think of it as three layers on the very surface of the base.
The Base, The Oil and The Scum.
If you move the scum slightly to one side underneath is the oil, likewise underneath the oil is the base.
If you actually taste the scum, its pretty grim and bitter, because like any scum its the impurities
created by the boiling process and in this case oil separation.
It really is hard to believe some people think it has a value and a good taste.
It's similar to a BIR Chef doing a huge pan of precooked Lamb or Beef, the first stage is to quickly
boil the meat to get rid of it's impurities (in the form of Scum) This is washed away, fresh water added
then the clean meat is precooked.
Each to his own, but that's my take on scum and why I don't want to eat it.
I also never make a base Garabi without a sieve to get the correct texture.

cheers Chewy