When I was very young, learning to sew, I don’t think I changed my sewing machine needle for 10 years! I kept blaming my bobbin and the machine for all my ugly stitches. I thought, if it doesn’t break, why change it???
Well, after decades of experience I can tell you that is the wrong attitude and philosophy. You should be changing your needles every other garment, if not every garment. It depends on how intensive your garment is, how much you are sewing on it. A simple two piece top does not use as much sewing needle use as a full length lined coat. If your fabric is fragile (= expensive), I would use a new needle. If you are using a sheer or silky fabric, a new needle. A dull or bent needle can wreak havoc on your sewing project. Here are some indications that it may be time to change your needle: Threads breaking, popping sound when going through the fabric (dull tip), skipped or uneven stitches, large punctures in the fabric.
Needles come in sizes (eg. 80/12). The American system uses numbers from 8 – 19. The European system uses 60-120. So 80/12 is the same needle depending on the country. The lower the number, the finer the needle.
Contrary to what your manual says, you can use any brand of needle in your machine. Most modern machines have a flat back upper shaft (the part that goes in the machine), though some are fully rounded. Check that to be sure you are buying the correct needle shape. I usually buy Schmetz. Janome and Jura machines come with Organ needles. Klasse and Birch also sell needles. I buy a full package of one size, not the multi packages. Over time you will know what needle you use most. I have a drawer full of 90/14 Universal needles I never use.
Since each type of fabric comes in different weights, there is no one-size-fits-all answer to the question of which needle to use for each type of fabric. Example denim: There are thin, lightweight denim fabrics, and then there are very heavy denim fabrics. The thicker fabrics will need a thicker needle. Always test the needle on scrap fabric from your project. Check the tension, stitch lengths and appearance of the seams.
My suggestion is to wait for a Spotlight sale and stock up on different needles. That way you are ready to tackle any project, and have back up when you need it.
Your machine probably came with a Universal needle 90/14. A Universal needle will get you through most garments. It is a very slightly rounded tipped needle, mid-weight shaft. I use an 80 for almost everything (cotton, linen, lightweight wools). 70 would be for Tana Lawn. 90 for thicker wool. But there are differences in other needles, even though they have the same size on the package.
Jeans: A Jeans needle 90 is not the same as a Universal needle 90. The Jeans needle has a tad sharper point, and a stronger shaft to get through multiple layers of denim (= canvas). Also, the eye of the needle is slightly larger to accomodate jeans thread, if you use that (I do not). I use this needle for jeans and any heavier canvas fabrics.
Topstitch: On the subject of jeans…. a Topstitch needle has a much larger eye than any other needle. They also have a stronger shaft. This is to be used with heavier weight topstitching or upholstery thread. I use these when topstitching jeans. But they can also be used on wool or mid weight fabrics. Not on sheers, though, as they would gauge a hole in the cloth (and who would topstitch heavily on a sheer fabric??).
Microtex: A Microtex needle is extremely sharp. It is at the sharp end of the spectrum, with Universal in the middle. It is the needle I use to sew lingerie and swimwear. I also use this to sew silk, fluid fabrics, chiffon, sheers, even cupro.
Stretch: As the name says, this is for stretch fabrics with lycra and elastic fibres. The tip is more rounded than a Universal needle. You need the rounded tip to not cut the cloth fibres when you sew. You may notice this on garments over time when the tiny ends of elasticised cloth stick out. That means the fibre has been cut.
Jersey/Ballpoint: They are basically the same. On the spectrum Ballpoint is the most rounded needle (vs Microtex being the sharpest). Next comes Jersey, then Stretch. Use these for stretch knits, or merino knits. Anything that is more stretch knit than elasticised knit.
Leather: Again, as the name says, for leather. But not faux leather! That has fibres like a cloth (here you need a Jeans or larger Universal needle). The point of a Leather needle is wedge-shaped so that it penetrates leather and other heavy non-woven fabrics.