Drained the oil.

Renewed the oil filter (meaning I changed it)

Filled with 8qts (data says it should take 8.5)

And then I see the dipstick indicates I've overfilled it!

And my oil pressure guage says the cars blood pressure is higher than my Grandmothers!

What happened?

 

If you let it drain for long enough and you are checking on level ground.

Could be that your filter is full of air rather than oil. Causing the high dip stick reading and or gauge pressure.

Leave over night and recheck.

You could slacken off the filter a little to bleed it (can be messy, be prepared) run car, turn off, slacken filter. Air out, oil in, oil leaks, tighten.

If that doesn't work follow above but with engine at idle. Have a helper behind the wheel ready to switch off QUICKLY.

If the car is overfull then you should drain some off, and recheck.

A word of advice for the future, when adding new oil, start checking around the 7 quart add point, and add slowly. Then when you have a good reading, run the car. Then stop allow it to settle and recheck, add more if required.

Don't just throw in the full amount right from the start.

After an oil + filter change my XJ40 4.0 always runs at a higher pressure reading than before the change.

I also seem to remember that the pressure switch is a known bad point and that later cars just had the simple Yes/No type switch that always gives a reading in the middle of the gauge.