I've got an '04 Outlander with about 40k miles on it. I change the oil regularly, usually with Mobil 1, but for the last two changes, I used Castrol GTX (for no particular reason other than to try it.)

Now I want to switch back to Mobil 1, but I was reading that if you switch from petroleum-based oil to synthetic, you should use a flushing oil first.

Should I worry about this after only two changes with the Castrol?

And if I do it, do I need to fill the engine with a full 4 quarts of flushing oil to let it idle or would one do the trick? (I was planning on using Mobil 1 0w40 to flush.)

Any tips are greatly appreciated.

Thanks,
Phil

I found this information at a site called the Car Bible.
Car Bibles website

Look partway down the page to see some discussion about changing types of oil.
You would want to talk to a trusted mechanic before you actually make the change.

Hope this helps

I was told that it is not a good idea to switch from reg to synthetic as it may eat or shrink the seals.