All 3 cars have poor mileage, the XJ6 should have gotten 18mpg.

I assume you are looking at the computer display for avg mpg. Have you reset it lately? How long do you let your car warm up in the winter? How long is your commute/average drive?

Unless you like to exceed 4500 rpm a lot, you shouldn't need premium gasoline.

Check the tire pressure, then check the spark plugs. The gap is .028 for single prong plugs, copper core. No need for iridium or platinum which don't last as long.

Does your car heat up rapidly, like in one mile, after cold start? If not you may have a wrong temperature thermostat.

Sounds like you have a shoddy dealership, or lazy mechanic. A poor functioning PCV will blow seals, but not impede mpg.
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Keeping it running is better than buying new