I don’t want to over-work this
issue, but I would like to suggest some caution (and/or some research on oil
properties) before rushing off to solve a cooling system problem by changing
the oil viscosity grade. The choice of oil should be made by the service
requirements of the engine. Certainly oil properties are not a simple
subject and it would take a book to cover all the factors. At the least I
would think that Mazda’s recommendation for the engine, for moderate to
severe service, should be followed.
Viscosity grade is only one of a dozen
or more measures of oil performance, and is a measure of how easily it flows,
at specific temperatures. The dual numbers on a multi-grade refer to its
viscosity behavior at a low temp (can’t remember if it is 0F or 0C), and
at a high temp (100C, 212F). A 5/30, 10/30 and 15/30 all have the same
viscosity at 100C, so at our operating temperature they are all very similar. But
the wider the viscosity grade range results in some compromises, especially in
petroleum oils, where long chain polymers are added to extend the range from
the base grade.
Among the other measures of oil
performance are film strength (high temperature shear), viscosity index,
surface adhesion, flash point, flame point, dispersant ability, ash content,
CCS rating, additives, resistance to temperature breakdown etc, etc. One
of the most important of these measures in determining the protection against
engine wear is the film strength. Film strength is directly related to
viscosity because the longer chain molecules that result in higher viscosity also
result in higher film strength. In petroleum oils, single grade
invariably has higher film strength than a multi-grade with the same high temp
viscosity, especially after they have run for awhile.
Premium synthetics have generally much
better film strength, and most other qualities, than petroleum oils. For
our service in an airplane, which I would rate as moderate to severe, I would
not want to use petroleum oil with a high temp viscosity (the high number on a
multi-grade) of less than 40 to 50, and would probably opt for a 20/50 in a
quality multi-grade. You can go to a lower high temp viscosity rating in
synthetic, so maybe 30 to 40 for the high number on the multi-grade. And
synthetics can be formulated in multi-grade without adding polymers, so a low
range isn’t as important, but the greater the range, the more of a compromise.
Unless you live in a cold climate where you may have frequent starts in
below freezing conditions; a 10/30 or 15/30 would likely be a better choice
than a 5/30. If I’m running a racing engine (severe service) I
might want 20/50 even in a synthetic.
Choice of viscosity also depends on
porting in the engine, bearing clearances, etc. The manufacturer’s recommendation
is a good staring point. I personally do not know what Mazda’s recommendation
is for the rotary in moderate to severe service. You would generally not
want to go more than one grade higher than the recommended for a stock engine.
I’ve drawn some of my own
conclusions, but everybody has their own. A pretty good, but somewhat
wordy reference is The Motor Oil Bible
that you can download free at www.themotoroilsite.com.
His rankings for the best in a 10/30
would be Redline, Amsoil, and Neo; in a 10/40 would be Amsoil, Redline, Mobil-1;
in a 15/40 Redline, Neo and Amsoil. But I would guess any of his top five
ratings in any grade would be excellent choices.
Just scratching the surface, FWIW.
Al