"Dan Olstad" wrote
the manual recommends straight 30W oil.
Why not a 20-40W or a 20-50W? The
manual says that's ok if straight weight is
not available but it is not recommended. Can anyone shed any light on this
for me?

Disclaimer: This is free advice and you get what you pay for.

My understanding is that the reason single grade oils are recommended for
boats is to reduce internal corrosion in your engine. Because a single grade
oil is thicker at lower temperatures than an equivalent multigrade oil, it
sticks to the bits and pieces inside your engine better after you shut it
down and things in there cool off.

Now, about those Ws. Your manual probably recommends an SAE30 and those
multigrades you're looking at are probably 20w40 and 20w50s, not
number-numberWs. The W stands for winter and what it's all about is the
temperature at which the viscosity rating applies. If there's no W, the
viscosity rating is measured at a high temperature, probably something like
212 degrees Fahrenheit. If there's a W attached, the rating is measured at a
cold temperature, like maybe zero degrees Fahrenheit. A 20W oil has 20
somethings (what that number actually means is kind of mysterious, at least
to me, so I like to just think of it as a relative thing) at zero degrees
and would be less viscous, ie lower number, at 212 degrees. An SAE20 (with
no W) oil would be more viscous, ie higher number, at zero degrees than a
20W. At any rate what 20w50 means in oil lingo is that at zero degrees
Fahrenheit it has the characteristics of a 20W oil and at 212 degrees
Fahrenheit it acts like an SAE50. They make all that happen by adding
polymers that wrinkle up or stretch out depending on the temperature, making
multigrade oils thin out less as their temperatures rise and thicken up less
as their temperatures fall than single ones do. This is great for cars,
especially in cold weather, because it gives you a relatively thin oil at
startup, so it circulates nicely around the cold engine, and then it stays
thick enough once the engine warms up to keep things nicely lubricated. And
since car engines are typically used frequently and seldom sit around in a
moist bilge, it's not a terrible problem that the oil drips off of the
innards relatively easily when the engine cools off.

All of the above may be total BS, but I did get it from a reliable source, I
think, but unfortunately that was so long ago that I don't remember who or
how reliable that source might have been.