Question about anti-freeze /coolant top off. Is this bad?

You'll probably be OK; but I'd keep an eye on it. Look for floaters or discoloration in the coolant. Any signs of a chemical reaction.

If I'm ever low on coolant and can't get OEM coolant; you can always dilute it down with distilled water. It's a better option than risking a chemical reaction. You just need to get the coolant corrected before a good long freeze.

 

That’s hard to do if you can’t get full strength Toyota (or Toyota approved) coolant.

 

If your just doing a drain & fill, the pre-diluted 50/50 mix should be good enough for the lower 48.

If your doing a system flush with distilled water, you need to buy concentrate. You won't be able to get all of the water out of the system. So if you try dumping premixed coolant in, you won't be able to get to the freeze protection concentration you want.

My brother did a coolant flush and change on my GMC truck. Previous owner had the green stuff in it. When I found out he used the 50/50 stuff, I tested the concentration. It was too low to take a 3-day ski trip to South Lake Tahoe. I had to dump out a gallon of coolant and add 2/3 a gallon of concentrate to get it where it should be. I'm lucky to live in sunny California, cause I could've easily blown out a freeze plug - during the two years that diluted antifreeze was in there.

 

Hypothetically, once you've flushed with water, you can never get back to 50/50, by topping up with 50/50. With each drain and fill you will get a little closer, but lordy...

Best tactic I can think of is in one of the links in my signature: "flushing out stop-leak...", starting with step #4.