Are you circulating 24/7 with the circ pump between the boilers? Your drop is normal, but we need more info. A series connection (hot from coal boiler to return on oil boiler?) 100' is nothing, but tell us more, and what circ are you using, and the pipe type and diameter to the oil boiler. If the question is related to using the same pump as the oil boiler, then yes, you need a dedicated pump between the two heat sources, and I'd get a 3 speed unit like the Taco 0015(old model was the 008R) or whatever pump curve you need.