My water reservoir has engine oil globules in the coolant. Is this serious?

Any time you have oil in the coolant (or vice-versa) you could be looking at an engine with a faulty cylinder-head gasket. Cross-contamination of the cooling and lubrication system suggests that the gasket has failed, allowing the two fluids to mix. Eventually, you’ll lose enough coolant (into the engine’s combustion chambers or oil system or both) to cause the engine to overheat. Or, you’ll have enough coolant find its way into the oil to dilute the oil and cause damage to the engine. Either way, it needs to be fixed and not allowed to deteriorate until the damage is irreversible.

Changing a head gasket is a fairly major job, particularly in modern engines with multiple overhead camshafts and all sorts of pollution controls fitted. But before you commit to this, there’s a workshop test called a TK Test that analyses the coolant for chemicals associated with the engine’s combustion process. If the test reveals these chemicals in the coolant, then you definitely have a gasket failure.