This Practical Machinist forum post by Lakeside53 says the following:
Look in the Quincy manual (you can get it online). The oil you use
depends of the temperature. If your compressor is "inside" and/or not
subject to freezing, Quincy recommends 30 SAE (ISO 100) non-detergent
hydraulic oil. Outside low temperatures -down as low as 10 SAE.
I need ...
Thanks to @ThreePhaseEel I dare say the problem is fixed.
Asking the right question - in this case "what make/model are your circuit breakers" is what led to the answer - a standard breaker was inappropriate for such a large inductive load. After replacing it with a "high magnetic" breaker it hasn't tripped once. I did not know such ...