Little and often is the key.
When I had my DM9 I lubed the bolt after every game. Just get a small amount of lube on your thumb and forefinger and wet the orings. You dont need a lot just enough to make them seal. If you cake it on all which will happen is it will get blown off and end up coating the inside of your barrel and cause bolt stick.
I used hater sauce lube but the standard lube will work fine. As far as changing o-rings goes only change them if you get a leak which you cant solve through normal re-lubeing.