What Oil do You Use, and What do Your Friends Recommend?

I just bought a Husqvarna riding mower that has a B&S engine on it with 5.2 hours on it. The guy that I bought it from said that the place HE bought it from said to use Opti-4 oil in it.
So what oil do you use, how much is it, what oil would you like to use but that is too expensive, what oil would you never use, what oil do your friends recommend, and do you have something specific to say about opti-4?

If you could just answer 1 or 2 of those questions that would be fine.

Thanks!