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This is true. The Canola oil lasts about 8 months when poorly managed. It's a lot more fragile than the expensive harder to find options like bitcool. Mineral oil is damaging to the hardware so there's another tradeoff in addition to the higher cost vs canola oil. I'm currently experimenting with a canola oil; mineral oil blend.
I use the spent canola oil for lubricant on chainsaws and gearboxes of farm equipment. I'm going to experiment with sawdust/oil logs to heat the house as well this winter. Basically, I'm getting value out of the oil even though I've had to do a couple oil changes on the hot water heater.
With the maple sap heater, the oil should last several seasons, when well cared for, which is similar to what you would expect from the RO filters too.
Canola oil is a great way to play around and learn about immersion systems with low up front costs. My plan is to do a larger central boiler system which would use the more expensive and long lasting fluids. I'm confident with what I've learned from using the canola oil that I'll be successful with the larger scale system.
Canola is clever because it's non-conductive and cheap. But how often do you have to change it? I heard vegetable oil goes rancid after a few months at 70C+ and gets gunky. Mineral oil lasts longer but costs 3x. What was your experience?