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2004 Mercury Grand Marquis - should I switch oil?

Car uses oil no smoking or leaks. Adding one quart a month. I use 5w20 Castries. Should I go to 5w30 and add lucust oil treatment or what do you recommend

That sort of depends on hpw many miles you drive a month.

Oil usage is measured in miles. How many miles before added oil and how much do you add?

Ray , it is a little difficult to give you an answer from your vague post.
Do you only check the oil level once a month ? How many miles do you drive before you add a quart ? What does you manual call for oil ? How many miles on your 15 year old vehicle ?

Look in the owner’s manual and see what it says.

This car has been on the market for many years and originally had a 5w30 specified.

To squeeze a few extra miles per gallon, Ford changed the spec to 5W20 on may of their cars, with resultant oil use problems since 5W20 is extremely thin and needs a very tight engine.
I would have no problem switching to a high quality 5W30 oil with this vehicle. Try it an see if it makes a difference.

I f you have a leak somewhere or bad valve guide seals the switch won’t help you.

If the engine uses oil, and there are no leaks, then the oil is being burned,

Add a can of this to the oil, and see if the oil consumption slows down or stops.

https://www.restoreusa.com/index.php

Tester