In terms of detergent vs. non-detergent, I'd stick with the detergent. It's
hard not to anyway, since almost all oils these days are detergent to
varying degrees. Keeping gunk in suspension so that your filter can remove
it is simply a good idea. In addition, it prevents the build-up of
deposits, not only in low areas, but on wall surfaces.
On to straight-weight vs. multigrade...
In the old days, when a manufacturer recommended a straight-weight oil for
use in a particular engine, that recommendation was based on the ability of
the oil to deliver adequate lubrication at the high ends of the operating
temperature and RPM ranges, while still being capable of adequate flow when
the engine was cold. Otherwise stated, what they really wanted was an oil
that would be viscous enough at high temperatures to provide adequate film
strength and oil pressure, and which would also flow at an adequate rate to
feed the bearings at low temperatures. With straight-weight oils, this
always resulted in a compromise.
When oil thins out at high temperatures, oil pressure drops. This is
because the thinner the oil, the easier it can exit the pressure system via
the bearings and squirt holes. The pump is simply unable to keep up with
the high rate of loss and still maintain full bypass pressure. If the oil
becomes too thin, it may not be able to keep the bearing surfaces
adequately flooded with oil and you'll end up wiping out bearings or
scuffing other surfaces.
If oil is TOO THICK, it will have trouble flowing into the small-clearance
areas, such as the bearing oil clearances (maybe .001 inch or less, give or
take), and fully saturate the bearing surfaces. However, this is generally
only a problem when the oil is VERY thick, such as when a cold engine is
first started. This is one reason why most of the wear and tear in an
engine takes place in the first 10 or 15 seconds of operation: it takes
longer for the oil to flow into all of these tight bearing clearances to
provide a lubricating layer. When cold, it also takes longer before oil
starts spraying out of the squirt holes and longer to saturate cams or
cylinder walls. However, even with a cold engine and very thick oil, once
pressure builds up AND the bearing surfaces are saturated, there is
generally no problem.
Even thicker oil has no difficulty flowing through the large galleys or
even the smaller passges to the bearings...it's the bearings
themselves...the tight-clearance areas...that create the bottleneck. This
is why when you first start a cold engine, the pressure may come up fairly
quickly on the gauge, but the bearings or lifters may still clatter for a
few seconds after the pressure comes up. This is true no matter where the
gauge is mounted. The pressure in the galleys will come up very quickly
once oil has displaced the air in the passages and it hits the bearings
(the bottlenecks). The problem is that with cold, thick oil, it takes a few
seconds for the oil to flow out and saturate the bearing surfaces, flow
through hydraulic lifter orifices, etc.
The advantage of a multi-grade oil is that it does not thicken as much when
it is cold, allowing quicker saturation of bearings and other surfaces. At
the same time, it is formulated not to thin out as much at higher
temperatures, so that it can maintain adequate viscosity and keep pressure
up at all points in the system. Multi-grade oils don't actually thicken up
at higher temperatures, they just don't get as thin.
Before multi-grade oils were perfected, racers commonly used heavier-weight
oils, such as SAE 50W, to provide adequate lubrication at higher
temperatures, RPM, and loads. However, they usually didn't have to start
their cars on sub-zero mornings, and if they did, they didn't put any load
on the engine until it warmed up.
Early multi-grade oils had problems with loss of film strength at very high
temperatures, but this problem has been largely overcome, and synthetics
are even better.
If anything, you will improve oil pressure to all bearings at high
temperatures and high RPM by using the proper multigrade oil, especially a
synthetic oil. Synthetics have a flatter viscosity curve over a wider
temperature range than non-synthetics. My own recommendation (no written
guarantee, of course...too many other factors) is to use a 5W-50 synthetic
oil to replace a 30W straight-weight oil. This will give you better
lubrication at both high and low temperatures.
I've seen the hot oil pressure come up from switching to a multigrade
synthetic on every vehicle that I've ever used it in and I've never had a
hint of a problem. Used car lots routinely put it in high-mileage vehicles
because it makes little things like connecting rod bearing knocks quiet
down when the engines are warmed up.
The other thing to keep in mind is that viscosity isn't everything. There
is not a direct one-to-one relationship between viscosity and film
strength. Synthetic oils have better film strength than other oils, even at
the same viscosity.
If you do use a straight-weight oil, use a grade thicker in the summer
(such as 30 in summer and 20 in winter). You might also want to avoid
running the engine at a high RPM or under a heavy load for a minute or two
after starting it. This is because even after the pressure comes up, the
amount of oil coming out of the squirt holes is limited and you could scuff
cylinder walls or camshafts.
Engines that are run at normal operating temperatures for long periods of
time, such as over-the-road trucks, benefit less from synthetic oils, since
their engines are simply not started up as frequently and the oils can be
selected for maximum full-operating temperature protection. This is the
main reason that some trucking lines have not yet made the switch to
synthetic, although others have. Most taxicab companies use synthetic oils.
They're also approved or recommended by GM for Corvettes, by BMW, etc.
If you take things a step further, you could get an engine pre-oiler, which
brings oil pressure up and saturates bearing surfaces prior to starting the
engine. If you really want to baby your engine, this is the way to do it.
It will probably make a greater difference in long-term durabilitly than
going to a high-quality synthetic oil.
One last note about synthetic oils. They produce far less ash and other
"coked-on" deposits than non-synthetics. If you want maximum protection,
use a pre-oiler AND synthetic oil.
I know...this has turned into another full-length novel. I'd better quit
now.
Regards,
Alan Bowes
(Salt Lake City, Utah)
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