A few months ago I found what I had been looking for for a while: An old sewing machine!
On a trip the Souther landfill Second Treasures shop (second hand store) I found a great looking old sewing machine. The machine is a Pfaff Synchromatic 1209. It is made in West Germany, so it is pretty old. It is a very sturdy machine with a body mostly made of metal.
When I got it didn't sew particularly well, the threads kept making a mess. I found the original manual online and realised that the previous owner had not threaded the machine properly (and made extra notches for holding the thread instead). But even after threading the machine properly it kept making really ugly seams. So I got out a few screwdrivers and took apart the bobbin and soaked it in WD-40. I cleaned most of the insides with WD-40 too, and changed the needle (after finding a secret compartment on the side of the machine).
After letting the machine dry for a few weeks I put it all back together. I googled how to correctly set the bobbin thread tension and then gave it a try. And it ran beautifully! Below is a photo of the difference in seam quality before (left) and after (right) cleaning and re-tensioning. Notice the huge lumps of thread on the before picture? Haven't had a single incident like that since the cleaning.
Not bad for a $10 machine from the second hand store!