Positive Displacement Pump
I agree with Milton Beychok in wondering about your perceptions on viscosity being the limiting factor.
What really matters is the flow volume and differential pressures you want. For low flows and low heads, diaphram pumps are suitable. For low flows and high heads, reciprocating pumps are suitable; gear pumps can also be OK. For medium to high flows and pressures, centrifugal pumps are suitable.
Vapour pressure can be an issue with all the pump types. If the pumps are running intermittently, during the idle periods the fluid can vapourize due to ambient heat. The vapour will accumulate in high points, and the pump casing itself or the inlet piping are potential locations. In this condition, the pump can easily lose prime, and wont start pumping when switched on. Reciprocating pumps handle vapours fairly well, and are more suitable for such applications. Diaphram pumps are also OK in these situations.