These comments are all helpful and may well cover the issue, but I did experience alarms due to a lube oil issue once, so perhaps consider this;
Consider the possibility of the higher temperature of the engine under load giving a lower fluid viscosity to the engine oil. As the engine is slowed, lube oil pump also slowed, in turn giving a lower- perhaps too low- lube oil pressure hence alarm. As engines age, routine lube oil pressure becomes less , so an older engine with slightly low oil pressure under load & in use can become too low as the lube oil pump slows with drop in rpm combined with hot oil causing alarm (or shutdown) . A lightly fouled prop can cause a slightly higher engine/oil temperature that could conceivably have a similar affect. If the issue is lube oil based, re check the lube oil used against engine recommendations, change the oil due to possibility of incorrect oil, oil contamination or oil age causing viscosity issues. The electric oil sensor alarm terminals can become corroded so these should be cleaned & checked, perhaps change the sensor or lastly rebuild the engine. But of course your alarm may be something else completely!
