WaIIy wrote:
I have Mercruiser 5.7's 1989 and boat on Lake Erie near Cleveland.
My mechanic recommends straight 30wt oil and that's been fine, today he
said a new bulletin came out from Mercury and it was something like 30
wt up to 60 degrees F and 40 wt for over 60 degrees F.

Now my mechanic says to use straight 40 weight.

What do you think?

Use the 40, if that's the recommendation.

The multigrade oils only thicken up when they get really hot (various
additives, plastics etc expand & change as the temp rises, but the base
oil is the lower claim, so a 20-40 is actually 20 grade oil with
additives to help when it gets hot), as in a properly thermostatted car
engine.

The trouble or one of the troubles:-) with raw water cooled engines is
they can't run the normal high (near boiling temp) thermostats, so the
engine & therefore oil might not get really hot & if it were multigrade
it "might" not thicken enough to give the engine proper protection when
worked hard.

Other end of the scale; some boat engines, skiing or heavy cruisers can
really put the engine to work & although the raw water low temp thermo
cools the block, still & all the oil can get hot, again 40 is better
than 30 in that case too.

K