I have always thought that lubricant is only really any use wile the pump is dry, after changing. Once there is water flowing that is sufficient. Some kits come with silicon grease. I generally grease the shaft a small bit of ‘keenol’ (kept in the engine bay for the stern gland), but do nothing with the rubber.
One thing I was always told never to use with rubber was vaseline, as its softens the rubber over time, but on an impeller it probably won’t there for long?
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