Quaker state full synthetic

Sep 3, 2005
Royal Oak MI.
Switched my grand marquis over to synthetic yesterday at 50 sum thousand miles. This is the car that does almost all freeway driving several hundred miles a day. Been doing Motorcraft at 5 or 6,000 miles. Wanted a full synthetic to see if I could extend that a bit. What do you think is a good oci with Quaker State full synthetic? Thanks!
Having your oil analyzed helps in determining an OCI.

People live in the "3,000 mile" and "5,000 mile" world. Others confidently recommend 7,500 miles while others drink the Kool-Aid and swear by 10,000 and higher mileages because it's written on the label.

Sure, highway miles on a fit engine with modern oils can go longer than Grampi did with his '53 Dodge but you won't know without sending a sample off.

RockAuto offers an economical sample mailer kit. Other more expensive testing is available for TBN numbers (remaining life, I believe).

As above, I wouldn't be surprised if you could do 10,000 mile OCIs but you won't KNOW without analysis.
Switched my grand marquis over to synthetic yesterday at 50 sum thousand miles. This is the car that does almost all freeway driving several hundred miles a day.
How does a 10+ year old car only have 50,000 miles if you're driving several hundred miles a day? The math doesn't add up. You could rack up 50,000 miles in just one year at that rate.
Somebody earlier posted this simple equation for formulating a realistic OCI;

crankcase oil capacity x50 x average mpg. For example my 2018 Ford Edge 3.5 holds 6 quarts and gets ~26 average mpg. = 7800 miles OCI. Engine oil life monitor will run to about 10K miles which I think is too far.

Makes sense. Maybe modify by adding *.8 if DI, *.7 if TGDI.
If you want to extend the service interval, I think a UOA is a great idea but I would not run out to 10k right away. I'd start at 7k and see how it looks, then gauge your next interval based on the results. Most national brand full syn oils are pretty cheap so I'd err with caution if you plan to keep the Marquis long-term, and be mindful of the filter you are using for these extended intervals.
Also, why QS specifically? It's a good oil, I'm just curious. M1 has annual rebates where you can get 2 5qt jugs for $14.xx each -- unfortunately the last one ended Sept 1. Pennzoil and Valvoline have occasional rebates, too. During October, NAPA has their Syn Blend (made by Valvoline) 5 qt for $11.99 (no rebate needed), it's not a full syn but it is a ridiculously low price on a very good oil.
I meen even Supertech is marketed to 10,000 miles now. As excellent of an oil as Quaker State is, I'd say 10K with easy miles probably isn't the end of the world.
That begs a question. Surely by now the 4.6l has a bunch of data regarding whether it runs cleanly or not. Then again, how any engine is treated BEFOREHAND will effect condition.

I agree with atikovi above by essentially streamlining the mental anguish and just driving the old girl and keeping it well oiled. Simpler.

The OP did say, "...extend [the 5-6,000 mi.] BY A BIT". So do a 7,500 mile OCI and spend the $11 once. Then decide how deeply into this hobby you want to go.

Eons ago I did 5,000 milers on Sis' '88 Jeep (bought used) and saw the oil drain cleaner each time. I upped it to 6,000 as that OCI didn't worsen the apparent dirtiness. The analysis I employed at that time was how opaque the oil looked in uniform glass tubes. (Don't tell the serious BITOGers).