unfortunately I don't have a manual

I ran out of oil and put a little less than 2 quarts or about 1.9 litres in and the engine says right under 10w-40 2.9 l which I can only imagine means that I should use 2.9 liters of oil.
it has a window viewer and if it's on it's kickstand it still reads less than the low line and upright it completely fills the window after the 2 quarts.

Now I understand that most bikes you are supposed to check standing upright, however if I ran out of oil and only filled it 2/3rds perhaps it ought to be checked on the kickstand?

 

Welcome to the forums....

I have never heard of a bike where you check it on the kickstand. Every bike I have ever owned, it is checked level. An 81 440.... I would say around 2-2 1/4 qrts with out filter change.

 

all bikes upright on level ground

 

Yep.

 

ditto..

 

also if you haven't started the bike yet, then the filter hasn't obsorbed any oil yet. if you were to put all 2.9l in the oil would initially be way over the fill mark, but as soon as you started it and the filter obsorbed some oil, it would drop back where it needed to be until next oil change.