Aftermarket inks at best will give different color results, and at worst can damage your printer components. Mixing inks is a definite no-no. You might be okay after all the old inks are flushed and new inks introduced, but the contamination of mixing the two is a sure recipie for disaster.
If you've already got the aftermarket inks flowing and your going back to OEM, get some cleaner cartridges and flush the aftermarket ink out of the system, otherwise you're just asking for continued problems. I've heard good stories about switching to aftermarket inks if the system is flushed properly first, and I've heard horror storries as well from just using aftermarket inks at all. The printer is manufactured to use inks that it was designed to use. You may get acceptable results using other inks, but not without a bunch of fiddling with your profiles, and you certainly can't expect any consistent results if you're blending OEM and aftermarket inks in the same lines.
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