Hi and thanks for using 2CarPros.com.
Honestly, it sounds like a head gasket issue, but an oil change should not have caused that. Often times when a head gasket goes bad, coolant will enter the combustion chamber. When that happens, the engine will emit a white/grayish exhaust. Take a look at this link that shows how to check for a bad head gasket. Check what it says and let me know what you find.
Also, here is a link that describes what I mentioned and shows pictures of it happening. Read through it, too.
Saturday, August 18th, 2018 AT 6:46 PM