I hate discussing oil, Everyone has their personal favorites, I was always a Quaker State user for 20+ years, swore by the stuff, Now I am a M1 fan. All of the premium oils are better than ever and with regular scheduled changes a car engine will last for quite a while.
"Red Line recommends extending the oil change interval to 15k miles, with a filter at 7,500. Sounds like they've got quite a bit of confidence in their product, but that sounds like a bit long to me, no?"
Michael, I'm in the camp of oil and filter change at 7,500 miles, My driving habits are
50 miles minimum each day on the Hiway to and from work. Very little around town.
Mobil use to advertise that M1 was good for 25K miles between changes, With that thought, you don't sell a lot of oil. Now they recommend following your manufacter's recommendations. 1. to be politically correct with the builders, i.e. Porsche recommends 15k for normal changes, 11+ quarts in Dry Sump 911 air cooled tank I think, My wet sump in the 944S2 holds almost 9qts.
2. Won't invalidate warranty from manufacter.
As far as not being worthwhile to use Synthetic, not if people are actually changing their oil at 2-3.5K miles. It would be hard to justify the additional expense.
If people are diligently changing oil at these intervals, are they doing it themselves or are they having it done at a quick change franchise? It would be expensive at that point. (Normal around Cincinnati is 20-25 dollars for this service)
Myself, I do my own, Use factory filters, (Don't get me started on my old favorite FRAM (SCAM now)and turn the oil in to a friend of mine who has a furnace that burns used oil along with conventional fuel oil.
Besides it gives me a chance to check out the underneath of the cars.
71 250 Coupe
92 400E Sedan
90 944/S2 Cabriolet