Switching between oils of completely different chemistry

[altaylar]

20 mile would be enough to mix the two oil properly, but no need to change it that early on. The mix, albeit not ideal, will still work well. Most of us don't keep using the same oil each time anyway. I change my oil every 5k miles, and i would do that regardless if its my first or second fill with that particular oil.

 

[Pablo]

Some engines/pans retain a lot more used oils than others. To impact the numbers to the level in the example- wow that seems like a marked difference. I personally have not seen such an influence

 

[AdmiralYoda]

As long as you don't switch to synthetic after using conventional you'll be fine as switching to a synthetic will make your engine leak.

Just kidding. Mixing and matching probably has zero noticeable effect on most engines in passenger cars. Severe service or race-duty......maybe but I doubt it.

Many owner's manuals spec 0W-20, 5W-30 or 5W-40 for the same exact engine depending on country. And who knows what the corner oil change place or even the dealer is using for oil today vs last oil change? Different brands, different viscosities, different chemistries...