OK to mix Transmission Fluid after Dealer didn't use WS

I didn't really ask for a flush specifically. The service advisor recommended a flush and I didn't know any better, so I authorized the service.

When I called them this week to get the info on the fluid used, the service department transferred me to the parts department because they buy the fluid for the service department. I really didn't have it in me to request that they explain the amount of fluid on an invoice from 2018. 10 quarts at $2.79 is well less than 4 quarts of valvoline Maxlife multivehicle ATF in today's prices, so I figure it was probably about the same then. Plus I am clearly never going back there again. I just wanted to know the fluid used to help me decide how time sensitive it is to get that fluid replaced again.

 

So I got the drain and fill done with Toyota WS at the independent shop today. The guy took a picture of the old transmission fluid being drained and in the drain pan. It looked like his personal cell phone, so I didn't ask for him to send it to me. It looked really dark, between the color of dark grey mud and a charcoal colored suit. I was hoping the Valvoline fluid would look a litte better, since people have said the WS looked pretty good when draining it a second time, even after much longer intervals than the first time. Glad I got it changed. I was planning on waiting till my next oil change in 6 months at 82,000 miles to change the transmission fluid on my 2020 prius prime with 73,000 miles, but the color of the fluid on the prius C makes me want to do it sooner. However, I want to wait long enough to notice symptoms of an overfill or underfill, just incase they weren't competent. Does anyone have an idea of how long it would take to notice symptoms of an overfill or underfill on the prius C? My wife puts about 700 miles/month on the prius C (mostly highway) and I put about 1,600 miles/month on my prius prime.

Just incase it makes a difference, the ICE is on for very little miles on my prius prime. I probably add about 10-20 gallons of gas a year to drive 18,700 miles. Last year I added ~12 gallons of gas. Not sure if this reduces the wear on the transmission or ATF at all. I figure the transmission is still working, regardless if the battery or ICE is being used.