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Replacing regular oil with synthetic

Hi all,

I do the regular oil changes in my 2001 Subaru TS Imprenza and have always used Castol oil changing it every 3,000 miles and the car runs great.

I just had my 120k service done and the shop used Kendal synthetic without asking me. I was told that once you’ve put synthetic oil in an engine you can’t switch back to regular oil, and being of the “If it ain’t broke don’t mess with it.” school I’m a bit annoyed. Can I switch back to regular Castrol oil, or is the Kendal synthetic really better?


You can switch back next time.

You can switch back to Castrol and I recommend switching your service shop. Sounds like it’s manned with total numbskulls.

It’s only better if you live in Alaska or Arizona or pull a heavy trailer. Forget the myths and nonsense about not changing back and forth; you can even mix the two!

You garage sould have asked your permission to put a more expensive product in. There is nothing magic about Castrol or Kendall oil; as long as the meet the specification that’s in your OWNER’S manual, the oil is OK.

The only reason you should be annoyed is that they charged you more. If you go back there, tell them you want regular dino oil that meets the car’s needs. If they balk, go somewhere else.

The idea that you can’t switch back and forth is just a myth that just seems like it won’t go away. You can switch back and forth as much as you want. A lot of people I know here in Minnesota run synthetic in the winter and regular oil in the summer, every year.

Thanks everyone.

They used to be a real good shop and they still have flashes of it, but they’ve gone down hill.

  • Modern oils allow you to switch synthetic and regular oil without problems. Just make sure your call does not specify synthetic, in which you must not use regular any time.

  • There is no advantage in chaining oil at 3,000 miles, unless you are going over the time part of the recommendation. Modern oils (both standard and synthetic) along with modern engine designs, mean those short oil change intervals are not needed and will not help your car live any longer than following the owner’s manual recommendations.

  • Yes synthetic is better, but, unless the manufacturer specifies synthetic, there is no advantage in using it. OK there is a tiny advantage as it usually will reduce friction a little and increase you mileage, maybe 0.01 MPG. ?