Do you think it really matters much if you use a different brand of top off oil other then what is in the sump? Let's say you are 4,000 miles into a 7,500 OCI and running QSUD 5w30 and add 1 quart of Motorcraft simi synthetic 5w30 Are you messing up the chemistry of the QSUD or after 4,000 miles does the chemistry of the QSUD even matter all that much?

I like to buy and extra quart of the oil that's already in my vehicles but it likely doesn't matter as long as it's quality oil. I think there was a similar post not to long ago.

The only time it really matters is in the winter when you're trying to maintain a zero winter weight. In your situation it would be good since you're replenishing the additive pack. Plus the oil is less dirty in the sump.

Nope everything is ok...just try to top off with similar oil base (mineral/semisyn/syn) and weight

If you threw some syn on top of mineral wouldn't you get semisyn? Why would that be bad?

Think of it this way, when you make the switch (couldn't help myself, sorry), you have about a top off's worth of oil in the engine. Now that is quantity, the quality of the remaining oil is the opposite of what you are about to do.
Still worried?
None of mine need topping off, the IS is finally fixed thanks to TSB, except for the HD. When I do top off, it's never the same, usually up a grade or two. With this manner, the more I top off, the less I need to. Your experiences may vary of course.
This is the perfect moment when the saying "oil is oil" applies.

It's gonna blow up.

In my Peugeot i have 5 different oils, of 2 different viscosities mixed in, and nothing has blown up

The most I do is top off a synthetic with another synthetic. I try to use the same weight but whatever I have on hand gets tossed in. Some oil is better than low oil, IMO.

top off oil?

For me, if your top off oil is the same API certification, I'll go thicker, not lighter, and use what I have handy or already opened. Everclear never.

Oil is oil there all use the same additives. What chemistry is there to mess up.

It does not matter. One of the primary functions of oil is to transfer heat. So more is better. Volume counts. Prolly more than trying to figure out if partially used chemistry will be negatively impacted ...

I have some old sm Napa syn From a broken case I carry around for top up. Lasting forever..

I have a big jug with the label torn off and "EURO MIX" sharpied on the side.

It's a mix of Pennzoil, Mobil, and Castrol 0W-40 oils with a bit of HDEO. All my leftovers go in there and I use it to top off anything and everything.