If you can't remember if or when the mower oil was changed, it is probably time to change it.
I used to change it every year in the fall, but the oil coming out still looked golden amber. So now I change it every couple of years, usually in the fall. I sharpen the blade at the same time so it's ready to go in the spring.
It really doesn't matter much spring or fall. The important part is that it gets changed every now and then.