Treat everything that says Mil surp or, pre 19xx as corrosive. Even if it's not it's better safe then sorry. If your not wanting to shoot the milsurp you could try the Sellier and Bellot ammo. A little on the expensive side, I got 40 rounds for $14 as opposed to my 1,000 of Ecuadorian at $63.
The reason it's labeled as corrosive has to do with the primer of the rounds. They put a salt content on the barrel and much of what the gunpowder touches. Now the salt isn't necessarily what's bad, but it attracts water and can pit/rust. Easiest way to clean is with warm soapy water, or some windex for the initial fouling, then just clean as you normally would.
With my M48 I usually slide the bolt out, dis-assemble it and place in a large bowl of soapy water while i'm cleaning the barrel. Once i'm sure the barrel is clean and the gun is re-assembled I take the bolt out, let it dry on a paper towel, clean and oil the pertinent parts, re-assemble and slide it back in. Next day I usually take it out again and check to see if there is any signs of spots I might have missed. I know, not necessary but i'm paranoid lol
Good luck