"Dan Olstad" wrote in message link.net...
As the season draws to a close I am thinking about fluid changes in my new
to me boat. I have 1987 Mercruiser 260s and 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? Thanks.-DanO

I would stick with straight weights unless you plan to operate in cold
weather.

20-40 and 20-50 are 20 weight oils. They have additives in them to
limit how thin get at normal operating temperatures. Consider that 40
weight oil will be pretty thin at 160-170F. But fairly thick at 60F.

20-40 will lubricate like 20 weight oil when its cold - it's thinner
and flows easier at lower temps. But 20 weight would be too thin at
normal operating temps. 20-40 flows like 20 weight at low temps
(because it is 20 weight) but is modified to get no thinner than a 40
weight at normal operating temps.

Drawback in boats: Do you run your boat everyday? I don't - I only
get out every couple of weeks. That thinner 20 weight oil will not
coat the internal parts of your engine as well during periods of
non-use. Thicker straight weights will stick to your moving parts
better and protect against corrosion, etc better.

You do want to exercise care when starting engines with straight
weight oil. Compare to 10-40, a 30 weight will be noticalbly thicker
and you want to make sure it's flowing before putting loads on the
engine. When I start my boat, I idle at about 1000 RPM until the temp
gauges start moving. At 1000 RPM, the initial oil pressure runs about
60psi. When it starts to drop to 50-55, I figure that it's starting
to flow easier and I am ready to idle out of the marina (depending on
which direction I go, I have at least a half mile of no-wake zone from
my slip). I'm running 283s. Those of you with bigger engines may
have different initial idle speeds - so don't assume I am doing
anything wrong because you run your 350 or 454 (or whatever)
differently.

Engines sitting for extended periods of non use are also why I won't
run synthetics. Too slippery - don't coat parts well for extended
periods. Synthetics do a poor job of hold contaminants in suspension.
The contaminants settle out easier leading to the increased
likelyhood of forming sludge.