You'll read and hear all kinds of oil recommendations and mixture ratios, so you'll have to experiment and decide for yourself what is best for your Solex.
The fuel and mixture ratio info in the manuals is generally outdated, since the modern oils and gasolines are different from those that were around in the past, but I would not say that the information is incorrect.
I personally use Lucas semi-synthetic smokeless, low-ash two-cycle engine oil, mixed at 40:1 for my older bikes ('60s & early '70s), 30:1 for my oldy (1940s), and 50:1 for my less old bikes (late '70s)...never a problem, but everyone that will give you their opinion will say the same thing.
Amsoil works very well, too.
And never a problem with the major name brands...Valvoline, Castrol, Pennzoil, etc.
Some info/opinions here: