1997 Chevy Blazer Oil Top off

1997 Chevy Blazer Four Wheel Drive Automatic

Hi, my oil cap says to use sae 5w-30. But I noticed that my step-dad used 10w-30, during the summer. I am due for an oil change but have a 3 wk wait for my (honest) mechanic. Which is why the wait, he's a rarity.I need some oil in it. So I was thinking I need to top it off. Is it ok to put the recommended sae 5w-30 on top of the 10w-30, since it's getting colder?
Also, does the lack of sae on the already used motor oil mean anything to me, pose a problem since the cap specifies sae?