I don't know where my head was at the moment, but I accidentally put 10w40 into a mower that already that had a little bit of sae 30 oil already in it. I realized it before I started the mower. Obviously, I know you should use sae 30 oil and that you wouldn't want to use 10w40 in the mower generally. The mower is pretty new, and I don't want to damage it, so I'm wondering if I'll be okay to use it now or if I need to just change the oil.