3k is way too often even on a gasser
I will have to respectfully disagree with you on that one.
When it comes to a gasser, I will stick to my 3k intervals.
Many moons ago I was a mechanic for 3 years at a Ma & Pa shop. We saw all makes and models (except diesel) there. There are allot of valuable things I learned there.
People who regularly go past the 3k oil change. Usually have the inside of their motor look like crap. So much build up & scaling on the heads & valve cover. Not to mention excessive wear on the top end.
there is CLEARLY a difference between certain makes of cars and others. such as Jap & German vs. American vehicles.
You can beat the crap out of a Jap & German vehicle and it will keep purring like a cat. Do that to an American car and it will see an early grave. you mentioned your Mazda and your brother's Audi A4. these don't fit into the norm.
Throughout my years of wrenching & the many conversations I have had with mechanics who where co-workers & my friends who are mechanics have all convinced me of one thing. change your oil every 3k and your motor will be happy.
I think I will take WisTDI recommendation and get an oil analysis done. This way I have proof about it.
P.S. Before some over sensitive nelly decides to make a degrading comment to me about the "Jap & German vs. American vehicles" thing. Don't bother. I am all for "buying American" products! However I also go by this philosophy as well. I work hard for my money, and I will buy the superior product to make "MY" dollar go farther.
In closing....
Its a crying shame we generally don't make superior products anymore.