oil or axle grease for turbine roof vents?

It depends. If, the bearings are sealed or just have a paper-thin slot, then oil. If the bearings are removable and can be taken apart, then re-pack them with grease. Although, if they're by a company like Lomanco, they might have a Forever Warranty and could be replaced entirely or just the turbines could be shipped back to have the bearings replaced.
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