One of the things I discovered when I had a 1978 A4 was that when I wasn't using Marvel Mystery Oil my compression was more than 1/3rd less than it should have been. The engine was difficult to start, especially in cold weather, and the plugs seemed to foul easily.
Using the oil, mixed with the fuel, the compression improved dramatically, which I believe was the result of the valves seating better. The engine started immediately, took just a few seconds to smooth out, got better fuel economy, and after six months of running nearly every day, the plugs looked like new.
Now, from my perspective, Marvel Mystery Oil is the best thing since sliced bread for the A4, and I will continue to use it as long as I have a boat powered with an A4. I love the engine, and have absolutely no fears running a gasoline engine in any type of boat, sail or power.
My current A4 was installed as a re-manufactured engine in 2000, it has a bit of blow by, but that's something I can easily address with a kit. The compression is good, and I suspect the blow by is from worn valve guides and seals. I put hundreds of hours on the engine during my recent trip to Marathon, FL and back to the Chesapeake's upper reaches. I fired it up on Saturday and it started instantly, despite the chilly weather.
I'll stick with the oil, and I highly recommend it to anyone who has an A4, or any other engine, for that matter. It sure works for me.