Last night I played with Umber and Lead White, mixing a tonal string to grade out a background. It was an on the fly decision so it's a bit rough here. After dropping in some initial values, I mixed the paints with a bit of my studio oil. My paints became smoother and with grab. Colors are more saturated. Dries in less than a day. I like it a lot.
Here is that same smooth finish and same lighting reflecting overhead fluorescents. (I changed my viewing angle to find the glare axis.) Is this a problem? I don't know. What would the glare would look like with multiple light sources?
Notice the lower left border in both photos, that patchiness. It's an area I'd gone over with pure umber paint, no studio oil. The first patch loses saturation and transparency; the second reduces glare.
No real judgements here, just observations.