Best is use the manufactures brand....
But still have to match the engine with the basic mineral based , or the semi synthetic, or the full synthetic.
 
There are 'cheaper' non manfacture brands  like valvoline, sierra etc.
Tried some of those yrs back but end up with more smoke , lot more soot around the prop and carbon in the chambers.
A few weeks ago have tried the sierra cause everyone saying how good it is etc.. and I mean everyone..
 My my tank is currently getting close to replacing the sierra back to manufacture band..
Those clouds of smoke on 1st startup at the ramp, and test run before a trip going thru the neighbourhood is just down right embasssing..

Lot ppl throw off at 2S being smokers..I wonder if it is because so many ppl cut a few bucks and go to the elcheapo oils.?
And thats what is seen at the ramps..

Crunch the numbers thu..
Brand name ball park $90 for 1 US gal (nearly 4L)
Elcheapo  about $15 cheaper.
 Thats about $4 per L 
@ 50:1 ave mix (older 2S)  that $4 less for 50L fuel used.

For us the ave trip was about 40/45L fuel, so a saving about $3, then split that 3 ways between crew, thats saving about $1 each per trip..

If smaller boat use say 10L 

Is it really worth using cheap oil?

Then chat to a few experienced outboard mechanics, they will have many a story of failures due to cheap oils.. no different to modern cars  if use cheap oils.

Like "Steps" was given a 4lt pack of Valvoline and yep soot n smoke. Switched over to the mercury red something little smoke and no soot flushing in barrel.$24 per lt worth it i think.