John
I understand most of your logic. I do firmly believe that Ultra and XP lines are superior with their construction and media for use with synthetic oils with extended intervals. I also use Mobil 1 in my GTO (15W50 - 1200/1300 ZDDP) and Camaro (0W40 1000/1100 ZDDP).
But are you planning to run your 389 for 10 - 15,000 miles without changing the oil and filter? As I change oil and filter every year, I'm totally comfortable that these filters won't suffer the issues inherent to the extended intervals.
If your plans are to change oil every 5 or more years (assuming something like 2,000 miles per year), then the concern over the filter makes sense to me.
I first started using Mobil 1 on my first brand new car - a 1976 Pontiac Ventura, so I've got a little experience with the product.
In 1979, after writing the insurance for my local Mobil oil jobber, he turned me on to using the commercial version - Delvac 1 - which I could buy directly from him in gallon jugs for about half the price. My 1979 Pontiac Grand Lemans 301 V8 made it 211,000 miles (before I sold it) on Delvac 1 with ACD oil filters and oil changes at 15,000 miles and filter changes at 7500 miles. Engine internals were never touched.
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