I use cheap dino Pennz 10-40 and 20-50 every 2-3k, Its still coming out fairly clean on my non- rebuilt 310k mi 3.slow.


But I use Syn in the sportsbike.


You should at least change the filter if you extend the oil change interval, Oil no matter type still builds particulates and if the filter builds up to much the oil will bypass it.

 

The only way to know for sure is oil sampling and testing. Companies with large fleets base it on test results so they don't waste money changing out oil too soon.


One of the instructors at the Caterpillar dealership I worked for used the oil lab for his change interval recommendations. They establish a base line and monitor the samples and make recommendations for YOUR specific vehicle. What you drive and how you drive it matters.


Last time I talked to him he was over 10 K miles on synthetic and oil did not need to be changed. Change filter per factory recommendation. OEM filter preferred.


Sampling will also tell you if you have an air intake leak or dirt getting by your filter, over fueling, normal wear or accelerated wear.


Most people don't have convenient access to an oil lab and change oil based on personal preference, factory recommended intervals or cave in to marketing scare tactics.


I have a hard time believing dealerships are replacing high volumes of engines because people went over 3k miles on their oil.


My Toyotas are about a 50/50 mix of city and highway driving. I run either Mobile 1 or Castrol (whichever is cheaper) and go 6 to 8k miles between changes. I use OEM filters and have had 5 of my trucks go over 450k miles.

 

I change every 5k miles using conventional oil.
I like the idea of sending out a sample of oil after using if for 5k miles to Black Stone Labs as a base line, comparing results with Lubricheck (saw it first on Andrew St Pierre White's 4x4 feature), and go from there. More frequent if needed or stick to 5K miles if results look good.
Reviews of Lubricheck look good, and it is .