A modifier clause after "one of the xxxs" – singular or plural?

I wrote a sentence of the kind "XXX contains one of the most powerful lubricants that prevent rusty gears, the main cause of machine failure." This is too wordy, of course, but never mind. The grammar authority marked this as needing singular instead of plural, prevent -> preventS, which I think must be wrong because I am trying to explain what these lubricants do, but there may be some other subtlety in the real example which I don't understand. Elsewhere I have seen stated that "one of the sisters" is singular not plural. A formulation of some general rules would be very much appreciated.

XXX contains one of the most powerful lubricants that prevent rusty gears, the main cause of machine failure.

Verb agreement depends on whether the relative clause belongs in the embedded NP with the plural noun "lubricants" as head, or in the upper one with "one" as fused determiner-head.

If you're saying that there is a set of lubricants that prevent rusty gears etc, and XXX contains one of them, then plural "prevent" is correct.