rmcinnis wrote:

Another important aspect of the rating is the fact that SAE motor oil is
rated at 100 degrees C. When you need the viscosity this is the
temperature that you oil is likely to be at.


"K. Smith" wrote in message

Yes & there is the rub with raw water cooled boat motors.



One thing to keep in mind is that inboard and stern drive engines run with
hotter oil temperatures than their automotive cousins, even though their
coolant temperatures are around 140-165 vs. 180-210. An automotive engine
normally operates at a much lower power output and rpm (2500 rpm @ 70mph)
and air flow is constantly flowing over and around the motor while running
down the road. That air from the car's volocity and/or the radiator fan
absorbs the operating heat emitted from the engine and the oil pan.

In a boat, an engine is located in a cramped area and also is sometimes
insulated to reduce sound and heat transfer to the other areas of the boat.
There is no 70mph air flow helping to cool the motor or the oil pan. A
marine engine is cruising at a much higher rpm and power output for hours at
a time. The internal parts of the engine, pistons, rods, bearings, etc. run
at a much higher temperature than the 140 degree cooling water, and the oil
has to asborb that extra heat. The combination of limited air flow and
higher power output in a confined space make the oil in a marine engine run
quite a bit hotter than in cars. You can test this out by using an infrared
temp gun on your boat and your car if they use similar engines. It is
especially true if you run the car and the boat at the same speeds for
several hours.

In a previous post, Karen was correct in a way. When a multi grade oil gets
severely overheated, the oil can thicken up depending on its additive
components. Today's mulit grade oils are much more heat tolerant than the
ones from years ago.

Bill Grannis
service manager