For the transmission application I'd opt to use the EXACT grade, as the
viscosity in this case relates to the "film strength" needed to keep
metal-to-metal contact to an absolutel minimum. If you used other than
whats specified you are vulnerable to scoring, galling and .....
ability to reduce wear of the gear tooth surfaces. A transmission or
gear set is considered an "extreme pressure" (lubrication-wise)
application and the oil viscosity (as specified) is VERY important to
keep metal to metal contact from ocurring... in this case the oil
supports the load between the gears and prevents metal to metal contact
..... only if its the specified viscosity. . In a transmission the
reduction in wear is a function of the exact lubricity specified. If
the trans. mfg. specifies 40W (Not 10-40W), then USE 40W .... only.

For the engine applications, the viscosity enables the bearings
(journal - crankshaft) to be lifted to the proper height within their
journals, so that the oil supports the shaft precisely at dead center
in the journal so that the shafting is in the exact cente r.... and has
equal and equidistant clearance (at the higher range of engine load) by
the pressure generated in the gallery/pump and somewhat by the
viscosity. Why viscosity is important is that viscosity (resistance to
flow) will help to support the piston load during the combustion
stroke. ........ However, If you are not 'lugging' the engine (low
rpm at HIGH load) then using a viscosity index of 1 grade lower than
specified ..... should NOT be a problem. If however youre running WOT
(wide-open-throttle) and high load ... then it becomes important to
match the specified viscosity grade (and operate at the specified
engine temp. ... as set by the thermostat.).
The short answer: OK to run lower viscosity, just watch the oil
pressure to be sure its 'similar' as when running the 'specified'
viscosity index oil.

In article , LaBomba182
wrote:

OK, so I'm sitting around with the captain and his engineer friend on the new
boat I'm project managing/co-captaining and we are trying to work out how to
run the same weight oil in the engines, gensets and trannys so we only have to
fill the clean oil tank with one weight oil and not carry any extra buckets of
different weight oils with us.
The CAT engines and ZF trannys can use 40W oil as per their specs but the
Northern Lights gensets only recommend up to 30W oil. The CATs could run 30W
but only at 86 deg. ambient air temps. We will certainly be seeing higher
temps
than that.
I tell the capt. and eng. that based on the ambient air temps we will be
seeing
even with the Delta-T fans that if it weren't for a warranty issue I wouldn't
hesitate to use 40W oil in the gensets.
The capt. and eng. disagree. They start saying that running a heavier weight
oil could cause wear problems and make the engine work harder over time. And
as
an example they compare it to what can happen if you use a heavy weight oil in
a high revving street/race car. I point out that this is a diesel not a high
revving street/race engine. They back off that point a bit but we still go
round and round in a civil fashion.
In the end we will end up putting 15-40W oil in the tank for the engines and
gensets (as per specs) and keeping some 40W in bottles for the trannys.

But other than the warranty concerns, I'm I missing something here?
Would using 40W over 30W in the gensets with of course changing it at the
proper number of hours and making sure that the ambient temps never get low
really cause any long term problems?



Capt. Bill