My question is how long are people generally comfortable going between oil changes when using a 20,000 mile oil and filter?
Some would go the whole 20k miles; some would not. Some folks use solid data and technical info; others use hyperbole and anecdotal experiences.
When you paint a question with a brush as wide as a house, you're gonna get a HUGE disparity in answers.
I figured I would change it between 5,000 and 10,000 miles. The oil life monitoring system on the MDX just recently indicated a change was needed at around 7,500 miles. My assumption is that monitoring system, at least in part, is based on the owner using the recommended full synthetic oil.
I believe your vehicle has an IOLM; it uses a host of parameters to track things like rpm, loads, temps, drive cycles, etc and then the algorithm spits out an OCI based on those conditions. The IOLM is predicated on using an oil that meets Honda's spec; it is not predicated on the base stock. Don't get caught up in the marketing terms; use a lube that meets Honda's spec, or a product that credibly can be believed to perform at/above that spec. Your owner's manual will have the necessary info.
I'm interested in what the scientific and engineering literature says about this, but I don't want to spend hours upon hours researching it. I am hoping some others on this forum have already studied this.
So ... you want an accurate answer to a complex question, and have no intention of putting any effort into it; hoping that others will do your homework for you? You'll fit right in here; there's lots of mythology and rhetoric to go around.
I do almost all my own repairs and maintenance on my vehicles, but I'm not an expert on this particular topic.
Doing your own repairs has nothing to do with understanding how to select a lubricant. Doing one's own maintenance and properly selecting a lubricant are not mutually dependent.
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I am a big fan of extended OCIs, when the pros and cons are properly understood. There can be great returns, but also great risks, in extending the OCIs past what OEMs recommend. Tools like UOAs and PCs are a great start, but you also have to know the history of the equipment (traits, pro and con, of your engine series). You'll need a good understanding of the lubricant base stocks and additives. And, personal experimentation plays into this. Etc ... One should NEVER jump into greatly extended OCIs. Rather, it's better to slowly, methodically increase the OCIs as you test the waters to track the progress of potential for success or failure.
I cannot stress this enough ... People who don't want to learn/know about the details of any topic, who aren't wanting or willing to get "dirty" by researching to the n'th degree, are bound to be confused and walk away feeling disillusioned. They want a quick silver-bullet answer, without any of the work. These folks are probably best served by just following the OE recommendations for oils and OCIs.
Honestly, the greatest concern for me would be the cylinder deactivation schemes on the Honda engines. I do not know the chronological history well enough to know which years are most at risk, but this is a technology that has never, IMO, proven to be helpful to the engine in the long term. It sacrifices longevity for the sake of fuel economy. It is unproven, IMO, at this point to know whether oil selection and oil OCI can dissuade the problems the VCM system induces.