Sergers have more moving parts than a regular sewing machine, and form a more complicated stitch. You need to use the needle system (remember needle systems?) that your serger is designed for.
Use the needle that your manual says you need to use.
This seems complicated, but it's less complicated than it used to be.
Old home sergers can use any number of needle systems. The most common are JLx1 and DCx1F. Your old serger probably uses one of these needle types.
JLx1 needles are round-shank needles, so you will have to be extra-carful to ensure they're installed properly. If they're inserted backwards or off-alignment, your serger won't form stitches correctly.
DCx1F are flat-shank needles (that's the "F" in the code). They're easy to install properly.
In the nineties, sewing machine companies started designing sergers to use the same 130/705 (or HAx1, 15x1) needle system that your regular home machine does. If you have a machine that uses 130/705 needles, congratulations! You don't have to stock another whole system of needles! You have the whole range of your home machine's needles available to you!
Also in the nineties, Elna had Schmetz design a new type of 130/705 needle for chain-stitching (5-thread serging) and coverstitching). This ELx705 (the "EL" is for "Elna Lock") needle had grooves on both the front and back of the needle, and a larger "scarf" (that cut-away portion above the eye) to make it easier for threads to pass and form chain stitches. ELx705 needles are only available in limited sizes (75/11, 80/12 and 90/14) and only universal and ballpoint (SUK) tips.
Industrial machines use their own needle systems, and you need to use what the manual says.
Check your manual. Always check your manual. Most manuals are going to say you can use size 75/11 to 90/14 needles. You might be able to use smaller or larger, but be very careful if you try. If you install a bigger needle, turn the machine by hand and ensure there is no clicking of any sort that indicates the needle is too big. If you install a smaller needle, run some test strips to ensure the needle doesn't break.
Most serger needles are universals, and universals will serve you well most of the time.Â
If you're serging knits, you will want to get ballpoint needles too. If you're doing coverstitch on knits, you definitely need ELx705 SUK (ballpoint/stretch) needles.
If you work with weird materials and your machine uses 130/705 needles you can use whatever normal needle type works best.
If you stick with the needles specified in your manual, it's pretty basic.
As with regular machine needles, serger needles are consumables. And a serger stitches faster than most home machines. Once again, replace your needles every 8 hours of use. If you buy bulk needles cheap, it's easiest to stick with the "new project, new needles" rule.
Coverstitch is really susceptible to dropped stitches if your needle isn't right. If you're having dropped stitches, try a new needle, a smaller/larger needle depending on your material, or a ballpoint (ELx705 SUK) needle instead of a universal.
There is only one case where you can bend the rules around needles.
If you have a combo machine (overlock and coverstitch) that specifies ELx705 needles, you can probably use 130/705 needles in the overlock needles clamp. Because the basic dimensions are the same, they should not break your machine, and will probably form a perfectly fine overlock stitch. If you're working with weird materials, having access to all the different types of 130/705 needles can solve a lot of problems.
You must still use ELx705 needles in the coverstitch needles clamp. If you use 130/705 needles for coverstitch in an ELx705 machine, you won't break your machine, but you will drop random stitches and ruin your work. Coverstitch is fussy, don't make it worse by using the wrong needles.