Crap idle and breaking up

Hi guys, I have had a code for a while that was for a cylinder 4 glow plug. With a brand new plug, brand new harness, and no bad fuses or anything, the code came back after I cleared it. Now I have a cylinder 1 glow plug code, and the cylinder 4 code is pending right now (literally just checked codes to double check)..

But the light has always been around. As much as I would like to get it taken care of, ever since I did my oil change it idles like ****. Nothing was different from any other oil change. Oil level was perfect, filter and gaskets all replaced, ect. But ever since then it idles like crap when it's cold, and usually smooths out once it's nice and warm. But it will still break up with moderate acceleration. And if I get smooth acceleration, it usually won't idle smooth.

Is the inside of the injector body waxing and causing it to stick? Do I have a bigger underlying problem?

I've seen this on older Powerstrokes and IDIs if it had a stuck injector or if the timing was off. But I don't see how the timing would be off, did the timing belt a few thousand miles ago, had timing double checked after just to make sure, and it all started with a simple oil change.

Again, fixing the CEL would be nice but only reason I even bring it up is because I don't know if that's the only way my car knows how to tell me that there is something else wrong, or if there is something stupid.. I've tried tracing wiring and find no breaks anywhere either

If you think you have an injector that's going out, you can check it with injector balance, group 013 in VCDS. Now what exactly are the glow plug codes? Numbers will help.

VW got weird with the glow plug numbering - many are labeled backwards (cylinder 1 is really 4) - and that would explain why you see problems with 1 after trying to fix 4 (really you should replace all, not just 1)

I know you should swap all plugs, they eventually were, making cylinder 4's plug swapped twice. All of them were saved and labeled though cuz I'm weird like that. When I finally got the motivationg to look back into my glow plugs, I learned that (my luck) I got two bad glow plugs, at two separate times, that were both installed on cylinder 4. I might have screwed up somehow and dicked the plugs myself, but who knows. Only reason I only swapped the one plug in the beginning is because living paycheck to paycheck made it difficult to get them all. Could only afford the one.

I check my glow plugs with an amp meter.
If they all have the same current I'm happy.