On my last engine, '02, 90 2 stroke, I had both (OEM first), and found the Seloc that iboats sells was very useful on a problem I couldn't solve with the OEM.
You can buy factory service manuals on eBay. Using the online manuals is a pain after awhile. I bought the factory service manual for the Mercury outboatd in my sig for around $40 or so and it was brand new.
I have the original service manual for my motors I have bought on ebay. I use Youtube, iboats and internet supply houses for supplemental information and diagrams. I am not an outboard mechanic, so a service manual tells me that just the bare minimum. Iboats allowed me to build my Whaler from a basic hull to a fun craft. I use my service manual to write notes in for quick reference. I have marked and copied pages so that I can do my yearly maintenance and occasional troubleshooting