I get two or three emails a week which run something like this one, which came recently..
"I was wondering if you sold the needles for the 319k.
Also I know that 12's and 14's are most commonly available. I wondered if it's possible to get any other sizes?
I sew a lot with denim and want to know what needles is best to use. I have been naughty and using schmetz leather needles in my machine. (Which work beautifully) but I'm scared about damaging my machine (because I adore her) or loosing or eye! " (all mis-spellings sic)
I generally reply in the vein of "Don't do that, please. Your machine is nice but sadly a bit obsolete, and it was not designed for this type of work even when you could buy other sizes of needles"
At this point I usually get an almighty snit from the enquirer, and indeed, today's was no exception.. "you're mean, you're rude, you're brusque and not helpful; I expect better, I expect nicer, I expect, I expect..."
Possibly, but if you ask an expert for (free) advice, then dislike the advice, it doesn't make the expert any less right, or the advice any less cogent..
And, I answer 100 emails most days - would you like an essay next Tuesday, or a reply today? You might wait a really long while for the essay, unless I like you, or I think you are a Nice Person..
I'm not your mamma; I don't have to soothe you, particularly if I think you are being silly...
Now, there's a lot of good reasons for not using the wrong needles.. The 206 x 13 type has a much shorter point below the eye.. (No, I don't know why Singer did this, but I suspect it was another of their "You can't get it elsewhere" games.)
This means that the regular needles (which have a longer point) will hit the bobbin case sometimes or more-often-than-sometimes, with results from scabby bobbin-cases to broken needles (You'll have your eye out with that...)
This in itself would be bad enough, but if you are working on "over-thick" fabrics (these are machines for dressmaking, maybe suits, not for car seats or boat covers or denim bags) then the needle is still in the top layer of the fabric when the feed is moving the cloth along, and this leads to all manner of scabs, breaks, frustration, and eyes-out.
None of which are good for you or the machine.
Unsympathetic? Moi? Too damned right I am unsympathetic. Or, I have no sympathy for you, but much for the poor old machine...
So, use it nicely, make frocks and patchwork, buy up needles when you see them. I have 12s (small) and 14s (medium) and absolutely nothing else.. The above correspondent assures me that "many sizes" are available on eBay, but I certainly cannot find them...