"Will an old stock 3800 with sealed yet out of date inks complete the setup
process with those inks?"
Probably. Make sure to agitate the cartridges well.
"Also, is it necessary to set up a 3800 with original Epson color inks
first, before installing and using aftermarket grayscale inks?"
No, it's not necessary, but it is a good idea, and third party manufacturers
often recommend testing the printer first with Epson products. Epson
cartridges tend to be more reliable than third party cartridges, and if
there's an issue, you'll get along better with Epson support if you use
their ink. Otherwise the first thing they'll tell you is to clean out the
non-Epson ink and install Epson ink.
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