I have never understood why people put 87grade gasoline in an engine that needs 89+. Saving 10 cents on a gallon won't even buy you a cup of coffee.

Anyway, the knock sensor error has nothing to do with the grade of gasoline you are using. It has everything to do with the knock sensor failing to do its job of retarding timing to stop the engine from harming itself with the poor grade of gasoline.
You will need to have the knock sensors replaced. While you are at it, have the PCV system checked for blockages, it is in the same area. And the vacuum line that connects to the intake manifold behind the PS pump should have the elbow replaced.

Klaus
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Volvoless. Sold the R. There is now a void...