mja

I took my S4 in for its 15k service this week. At its first servicing at 5k, switched over to Mobil 1. Took it in at 10k for another Mobil 1 change. This time the idiots put regular oil back in, and can't work me into their busy schedule fast enough to fix it. They say this won't be a problem (e.g., you can get away with it once), but I always thought once you go to syn, there's no going back.

Anyway, I want to change the oil back to syn myself. My question is do I need to change the filter too? The filter is brand new, and since it sits on top of the engine, I'm assuming when I drain the oil, it will empty leaving just trace amounts of the regular oil in the paper of the filter.

Let me know your thoughts.

DrGP

That leaves 4 qts of mixed oil in the system, regardless of what you put in. Of course, if you switch to synthetic the % of non synthetic leftovers continues to decrease!

CT04S4

and there is around another three qt's left in the engine. If you are changing the old out you can only change 9-9.5 qts of the 12. Another change later on and you will get out more of the old after it has of course mixed with the new.
I would be upset with your dealer on a few levels. One may be what Mobil 1 are they running? Only M1 0W-40 meets API SJ/SL and ACEA A3- two big requirements for our engine. There is no M1 that meets these two requirement PLUS has the other manual requirement of 0W-30, 5W30 or 5W-40. So my choice is M1 0W-40. It covers a broader temp spectrum and meets all the other requirements. Bet you dealer isn't using it, mine wasn't.
I'm going through the same thing, draining out an oil I don't want to run and going to the M1 0W-40. It will take a few changes until the old stuff is effectively all gone.
I'd recommend calling Mobil 1 tech support directly for questions about mixing oil types. They are a good resource.

rmarin2

What size oil filter do you use?

JDBlueAudiS4

You can go from one to another without any issues. Would I do it, personally no I would rather run the good stuff all the time.

mja

the filter moot.