You should assume the machine was set for sew-it-all type thread weight and go from there. Set top tension to 5, test sew two layers of medium to light weight cotton fabric (quilting and shirt type thickness). In theory it should stitch perfectly. For thicker threads you likely have to loosen bobbin tension.
If you see tendencies to loops under neeth the fabric, I would guess it's too loose top tenions, if you see tendendencies to loops on top of the fabric it's too tight tension. When diverting too much from sew-it-all type thread, (like top stitch) you might have to adjust bobbin tension. If the bobbin stitch looks slighly off to one side, or like the top thread isn't tightening it properly it's usually too loose top tension.
Don't use WD40 other than for dissolving dried up oil and grime, rather use basic sewing machine oil, Triflow or Finish Line Ceramic Wet Lube, the last two have added teflon for extra smothness and minimal friction. They do a better job than WD40 in the long run. Spray can oils are too thin to work optimally.
Last edited by Mickey2; 04-09-2016 at 12:43 PM.