I have used Mobil 1 synthetic oil in my cars for the past 20 years. I
think it is great stuff. It seems to help the engines last forever and I
only change it every 10k-15k miles. My wife and I together averaged
about 70k miles per year for the last 10 years. Changing the oil every
3k miles would have meant an oil change every 2 weeks on one vehicle or
another. During this time we have had 5 vehicles pass 250k miles each.
They are still running and none have suffered internal engine problems.
They all passed Califrnia emissions tests and used very little oil. My
daily driver BMW 325E uses a quart every 8K miles, original rings,
valves,etc. I use 15w-50.
80 BMW 528I, 300k+ (sold)
72 VW Westy, 250k+ (sold)
85 Mazda 626, 260k (sold)
86 VW Vanagon, 283k ,still have
86 BMW 325E , 300,244  (turned 300,000 2 days ago)