Left twist, or Z twist thread is, generally speaking, better for your machine since the twist plays nicely with the twists and turns the thread must following as it winds it way from the spool to the needle. Not all sewing thread is spun with a Z twist however. Mostly it is a european thing. Threads made in US, Mexico, and South America are (I wont say ALL because I have not seen every thread made) generally spun with a S or right twist. Right twist threads work much better in hand work because of the hand movements involved in stitching.
HTH
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