I wrote:
Beyond the trace of viscosity (present in every fluid that isn't a superfluid like helium 3) that is necessary for lift, lift generally decreases the more viscous the fluid, with all else equal. I'll see if I can dig up a graph.
Difficult to find anything explicit in the literature. McCormick has some plots of Cl vs AoA at various Reynolds numbers, but they're not superimposed so it's difficult to see the effect clearly. But it shows a flat plate giving a lift coefficient of about 0.75 at Re = 420,000 but only about 0.55 at Re = 42,000. So a 10-fold increase in viscosity causes a small decrease in lift at the same AoA.
Abbot and von Doenhoff also have plots at Re = 3, 6 and 9 million. It's clear that the stall happens at a lower AoA for higher viscosity (lower Re) but it's very difficult to deduce the effect of viscosity on Cl at low AoA.