Negatives to gear oil in motor oil?

[Hammerhead]

Let's break this down...

(10-19-2021 )Redland1 Wrote:  Are there any negatives to adding a gallon of gear oil to motor oil?

Yes there is.
There are different types of gear oil, some have chemistry that may harm the soft metal components inside an engine, other's will not.

(10-19-2021 )Redland1 Wrote:  I just did it for the first time and honestly have been thinking about if it was a good idea or not. Can it potentially cause any damage?

Yes it can. @Chamberpains is right, if you don't know or understand the cause and effect, then logically you shouldn't have done this, that's why your subconscious is screaming at you, it's concerned! Don't do sh*t like that on important components without understanding why you're doing something.
Follow-up questions;
What kind of gear oil did you put in? (The type, as in GL4, GL5, etc.)
If you don't know what kind. What brand?
What weight?
Do you know the part number?
We can't say one way or another if your choice was good or bad without this info.

(10-19-2021 )Redland1 Wrote:  Has anyone had any problems with it?

I can't speak for others, but I haven't and I've been adding a gallon of Sta-Lube 85W90 API/GL-4 (Part #: SL24239) in my engine during oil changes for years.
The only problem I had was that I could smell the aroma of hot gear oil, now I'm used to it and don't care.