Switch To Conventional Oil?

Mobile One finding every leak is what I've always heard, (and that has been my experience too, the couple time I tried Mobile One. But that was years and years ago).

maybe try an oil change with Motorcraft semi-synthetic or something like that and see if the makes any difference?

You go up to 10w-30 in these engines, make sure it's still synthetic though. I've ran 0w, 5w, and 10w in my 4.6 mod. 0w in the winter . And the thicker oil in the summer. All with no problems. Now these vortec 5.3 liters; by GMC, do not like the 10w, starts engine valve train knock. Nice to have an all purpose motor.

Try that Lucas addictive.

I run Shell Rotella T6 5W40 (yes, "diesel oil") in my 2010 STI and am tempted to run the stuff in my MGT on its next oil change. If you're burning oil and have cats though, this may not be a good idea as the elevated ZDDP in the Rotella may injure the cats over time...

For tired valve guides you might also try one of those "old school" tricks and add some automatic transmission conditioner to the oil. Those additives work to swell seals slightly giving at least temporarily better sealing. Not sure if the material used in the seals of the 4.6 would respond to that stuff in this way though.

If you're adventurous you might think about replacing the seals for each of the 8 cylinders. You could also check the valve stem to guide clearance (just wiggle things) to see if you need more extensive surgery.

http://www.stangnet.com/mustang-for...ple-that-have-done-it-engine-on-stand.833015/