When I repair a machine I use triflow oil on any metal to metal rotating part ie back and forth. On metal gears I put the tri flow grease. You can use it on plastic gears although sparingly. the nylon gears were design for quietness and were not recommended for grease. You want to get the old grease off as best you can. If it goes up and down or side to side then oil. If it continuously rotates then grease. Exception those bearings that have an oil hole vs a grease cap.
Doing a google search for the brand plus model number for a user guide or YouTube videos might find what you are looking for. That it is a Necchi doesn't supply the model number and of course models differ. On a 537L model, the large knob on the right side about halfway between the top and bottom of the machine adjusts stitch length. The numbers may range from 0 to 5 with 0 meaning no length and 5 being the largest length. Tension, stitch length and stitch width are all controlled by small dials on the front of the Necchi machine. If your stitch length is set to 0, the sewing machine will not feed the fabric. Incorrect tension will cause poor stitch quality. You must consult you machine's user manual to know how to adjust the tension as the procedure differs among machines. Here is a general guide, however:
How to Adjust the Tension on Sewing Machinehttps://www.youtube.com/watch?v=i-7bnb_6EBU Necchi
if you have a drop in the thread should fall off to your left hand when holding it up. but if itis a front loading type the thread falls off to the right.
Yes quality of thread could definitely affect sewing, as will a blunt/old/wrong size needle for the material used. Also must have same thread in bobbin and needle also corect thread and needle size for material sewn plus correct tension for that material too. Could be lint or tangled threads underneath needle plate too.
