@pegasus2 - If you haven't used tretinoin before, I would use a small amount on the other temple area (assuming you are only using SAG in one small area at the moment). Within 2-3 days of tretinoin gel (lowest concentration) first application, you will likely get red & irritated in that area. This will give you an idea of the effect you'll get. You can then decide if you really want to use it on the SAG temple. Once an area is irritated, you'll have to stop using anything in that area until it calms down. It will also likely peel. Might even have to use a bit of hydrocortisone to calm it down. And, the reservoir effect of tretinoin I believe is 3-4 days. Eventually, your skin will develop a resistance to the irritation, and, won't get as irritated. Even if you're only going to use it once a month, I would first test it on the non-SAG side. Also, if you do use tretinoin, don't go out in the sun without some kind of sun screen. Tretinoin makes your skin very vulnerable to uv radiation.
I've never used it on my scalp due to the fact that it causes hair loss for some people, I only used it on my face. I haven't been using it lately, but I would apply at night and never had any problems with sensitivity to sunlight. I've read that's a myth anyway. https://www.byrdie.com/retinol-cream-myths