Signature 50 weight thread (which is thicker than the Aurifil for some reason) and it sews beautifully.
You would think a dealer would know that Aurifil is 2 ply 50 wt and the Signature is 3 ply 50 wt. which means each one needs a different size needle because of the ply thickness. If the dealer did not know the difference then I would be very cautious about any advice given.