1989 mariner 50 hp outboard idle problem?

hello. I have a 1989 4 cylinder 50hp outboard boat motor. It runs fine when you give it gas, but when you put it back into idle, it will run for about 20 seconds and then will kill out unless you give it more gas. What could this problem be? Also, is there any other oil for the engine other than the lower unit oil? Thank you