If you have thread loops on the underside of the fabric then the top thread is not under tension at all. If the bottom tension is loose, it will be looping on the top of the fabric.
So, raise the pressure foot, and rethread the top from the spool to the needle making sure you pull the thread into the tension device. To test, lower the pressure foot and tug the thread below the tension device, it should feel firm. Tension dial should be fine at 5 or between 4 -6.
Don't mess with the bobbin tension, it should not need to be altered.
If you manual does not mention oiling at all, then your machine probably has nylon gears and maybe an oiling wick that is topped up by the technician during service. If this is a top loading bobbin then you probably still can oil into the wick under the bobbin in the round bit that goes around (rotary hook), there might be a hole in the middle? If yes, just put a drop of oil into it AFTER BRUSHING OUT THE ROTARY HOOK AREA well. This is just as important as lint will dry up any lubrication in the machine, then matt into a felt and cause wear and malfuntioning.
The stitching issues sound more like threading problems. Here are my recommendations for good stitching. Good quality thread, keep it in sealed containers out of sun when not in use, it will dry out. Don't buy the cheap 5 rolls of black thread in the bargain bin, fine for hand sewing and tacking but won't do your machine any favours.
Clean the tension discs regularly, clean out the rotary hook regularly with a little brush and try to get the lint out of the machine, don't blow it further in with compressed air. Check your manual for how to clean the tension, on your machine (if the image above is correct) I can't see a tension dial. But normally, you turn tension to zero, then floss between the discs with a selvage edge of clean cotton waste fabric. Dye particles and lint from thread will shed and build up in the tension, causing issues. If your manual doesn't show you how to do this, then go back to your shop/dealer and ask them how to clean this part of your machine. Do it regularly, it really helps. And oil as per the manual (although it seems this doesn't apply for your machine). However, machine will need professional service regularly to address lubrication inside the workings so don't forget to take it in regularly for this.
Put a circular piece of felt on the thread spool holder so that the thread spool sits on this, it helps the thread spool to turn smoothly.
Change your needle every 4-6 sewing hours, right size and type for the project at hand. When you start a new project, put in a new needle. I can tell from the sound my machine makes if a needle has gone blunt. Have a selection of needles on hand so you aren't tempted to just use the one in the machine.
Take care when winding bobbins that you wind it smoothly and evenly across the whole bobbin, keep gentle tension onto the spool as the thread winds off to ensure this happens. Give the thread a nudge with your fingertip as it goes onto the bobbin so it winds top and bottom, not just in the middle.
Always cover your machine up when not in use, keep out of direct sunlight or damp. Put a piece of waste fabric under the pressure foot between it and the feed dogs when finished using it.
For good Sewing machine info on line I like
www.sewing.about.com, there is good threading up, needle info and how to maintain the machine. Machines are all different but once youve mastered the basic funtionality and what parts do what, they all function in similar ways. The most important part is the needle, it does the sewing, everything behind it is just the machinery making it move.
