Proper lubrication

I had a conversation with a mechanic the other day about the proper oil to use in a JLG 600s with a deutz engine. We always use 15w-40 oil and he said that wasnt correct. So i looked it up on JLG's website in the service manual and it says to use SAE 30. Now i am confused and not so sure we are doing the right thing. What is the right weight of oil to use? We run these all year long so the temps will vary as much as Ohio does from an average of 85 in the summer to 20 in the winter. While i am asking..... what oil should go in a gas engine in a lift? 10W-30? What about hydraulic oil in a JLG brand lift? Always have used AW-32 in the past. Thanks!