There is a wonderful tradition at speakers corner in Hyde Park, London, at which a speaker stands upon his/her "box" (meaning whatever contraption he/she comes up with) and speaks until they are done, interuped only by "hear hear" or "balderdash".

Here I go.

I have a '73 Volvo 164E [3L inline six with the same miserable Bosch electronic fuel injection which MB used in it's cars...{which in my car happens to work v. well}] that my family has owned in various places around the world [including Italy where oil turns RED as it ages] and we assiduously changed the oil every 3-5K miles with "a" 20-50 as per manual.

I have a '86 Porsche 911turbo in which I only use Mobil 1 20-50 [again factory rec] because of the superior high temperature stability [400°F] which can be achieved by the turbo at spindown with engine off] ... it is a fine weather entity and gets about 3000mi/year and I change the oil every spring.

I now own an '83 300D-T [145k] and I change the oil with diesel grade 15-40 every 3-5K.

RESPOND TO THIS!!!!!!!!!
Certainly you all must agree that both the qualities and grades of oils have changed over the years as well as manufacturing techniques, tolerances and materials. Doesn't it make sense that there is NOT, in fact, one answer to this question??

I have my own questions... {rhetorical perhaps}

1] What place does 0-30 synthetic have? [Mobil 1 no less]
2] If you are change-o-philic, does it really matter whether you use synthetic [high temp example excepted]?
3] If people are so concerned about mixing, why would manufacturers [including Quaker] produce blends?
4] Does any sane person change the oil and not the filter?
5] Why does Italian oil turn red? {the Italian mechanic I posed this question to responded, "you would too if you ran around in a hot motor for 7500km!" but I considered the answer anecdotal and humerous rather than scientific}

thank you for your patience and I look forward to the continuation of this post