I've done the following:
In my '90 4Runner 3.0L, I used dino Pennzoil and Fram filters at 3,000-4,000 mile change frequencies. I towed a 3,000 lb trailer frequently. At 205,000 miles I sold the car to a friend and it used no oil. Last I heard he had 375,000 miles on it and was having no problems.
In my '00 4Runner 3.4L, I use Mobil1 and Fram filters at 8,000-10,000 mile change frequencies. I tow a 2,000 lb trailer. I have over 151,000 miles on this car and it does not use oil.
In my '07 Mustang GT 4.6L, I use Amsoil oil and filters at 8,000-10,000 mile change frequencies. The car has over 27,000 miles with no issues (but give it time, it's a Ford).
In my '08 4Runner 4.0L, I just did my first oil change at 1250 miles with Mobil1 oil and filter and will be changing it at 8,000-10,000 mile frequencies.
I've had 5 other cars (mostly Toyotas) that I've used synthetic oil in with 8,000-10,000 mile oil change frequencies and have gone beyond 200,000 miles and only had one that used oil; a whoping 1/2 quart every 8,000 miles.
Say what you will but my experience speaks for itself. I'm comfortable at these oil change frequencies.