I see this topic come up time and time here and elsewhere and thought I'd share the data points that I've come up with in my time working on cars as a DIY mechanic. This report covers two oil changes on my good friend's '98 Tacoma with the 2.4L I4. He treats this car like a beater (which is pretty much is getting close to) and is, let's just say, not very good at tracking maintenance on it. I've borrowed the truck to haul some things in the past so I offered to do an oil change for him as a trade if he bought the materials. This change is the oil analysis done at 125k miles after 10k on bulk conventional and a Toyota filter done by the local Toyota dealership. I called them up and they say they use Havoline. I was impressed with the results of that oil analysis especially because I believe the "Universal Averages" were based on something around 5500 miles. I offered him two choices for his oil change: go with what was cheap (Wal-Mart's Supertech) and change it around 7500 miles or switch to Pennzoil Platinum with the caveat that he ought to change the filter halfway through the next 10k mile oil change interval due to my theory that this engine could look pretty dirty inside after 125k of abuse and the synthetics’ tendency to act as a stronger dispersant would loosen up deposits and put them back into circulation, overwhelming the filter. The recommendation to go with synthetic was based on the thought that this poor truck would likely be subject to extended oil drain intervals and better oil might help him prolong the life a bit. Anyway, he went with Pennzoil Platinum but bought the basic Fram oil filter (cardboard end cap). I reiterated that I HIGHLY recommended changing the filter after a few thousand miles.

Well, it was last summer and the topic of oil analysis came up while we were shooting the breeze during a break at work. I asked him if he had ever switched out the oil filter and he chuckled and said, "nope, totally forgot!". Neither of us remembered the last time the oil was changed and he doesn't keep a maintenance log like I do so we pulled up the oil analysis and saw the date/mileage. We walked out to the parking lot and had a look at the odometer and saw it had been 18,000 miles!!! Same oil, nothing added (he hadn't even checked it), same filter. Keep in mind he drives this car less than 3 miles each way to work, goes home for lunch, etc. and happens to like to leave it running at idle for ten or more minutes at a time when it is hot out so the A/C can cool the interior.

The bottom line is that even in this situation with a marathon run of 18,187 miles on the first oil change from conventional to synthetic on an engine that has been neglected with the cheapest oil filter generally available the oil analysis looks amazingly good. You can tell that the oil was saturated with insolubles and that it had started to thicken (but was still within grade for a 5w30) but it continued to do the job.

Please note that I'm not advocating treating your car like this. I've posted this just to show folks that, in this particular example, doing everything opposite of "conventional wisdom" produced no ill effects.

good thing he switched to pennzoil synthetic..not sure the walmart oil would have held up as good.

It would not have. I think 5w30 Supertech is made by Shell (Pennzoil, check the MFR code on the jug to verify) and in the limited reading of other's testing I've done, it's similar to your basic yellow bottle Pennzoil conventional. It's generally a good value if you run conventional. There's nothing wrong with it but it probably would perform along the lines of the Havoline that was run 10k miles....for the first 10k miles. Once the TBN was shot and the TAN kept climbing I expect it would have started to look bad rather quickly, within 2-3k miles later if I fathomed a guess. Then you've got essentially septic oil running around that's not doing it's job which could snowball to high wear and potentially failure from there. 18k would have been ugly.