A lot going on here. You should not have any puckering, it can be related to tension or I'm thinking fabric feed.
There are a few things related to needle size; one is matching the eye to the thread, it has to run through easily. Another is getting a thick enough needle for the fabric; sturdy and stiff enough not to bend, and with a large enough groove on the side to accomodate for the thread. That's why upping a size or three for heavier fabrics often helps, similarly with very densely woven fabrics.
When adjusting tension, go for a medium to ligth thread weight (sew-all); I'm not sure what your Gütermann cotton might be, I guess they have selection of them. If you stick to quilting and one particular thread on this machine use it. Use two layers of cotton material for test sewing and adjusting and later on go from there. Set the tension between four and five, and if you still have loops on top, tighten bobbin tension. Make sure the thread is in the spring, and yes messing with a brush or smomthing in the bobbin area can upset the bobbin thread, even though the bobbin case in model 15 is more stable than drop in bobbin cases.
You can over oil, but it will only result in a wipe up job later on. When it's been a while since I've jused a machine I usually give it a second round oiling. These machine have decades of history, and if there's any stickyness or slow turning parts it needs repated oiling the next few days. You can give the screws in the throat plate a drop of oil now and then too, they will eventually free up. There are
clever screwdrivers to better access the area under the arm, it helps a lot when you can get a 90 degree angle (straight down on the screw). There's similar bit sets available too, you might have one in the tool box already.
Regarding the bobbin tension, there are tension gauges for this. You can get by fine with out one. There is a clever lady with a blog called Archaic Arcane, she has a few videos on how to get the tension right. One of the tricks is to weigh up a bag of beans or rice, and use it as a tension gauge. It helps a lot when the thread is out of the medium sew-all weight range.