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>> ...on the warm-up/ cool-down procedures you refer to?
As a ritual, I do no heavy acceleration 'til my oil temp reaches 150°F, as shown on my aftermarket VDO guage, which may take 10 miles or more depending on the ambient temperature. Since most don't have this indicator, I would suggest driving very moderately until well after the water-temp guage reaches its normal level. The water heats up to operating temperature far quicker than does the oil. This allows not only the turbo bearing but the rest of the drivetrain internals to get up to operating temperatures.
The cool-down is more important as it relates to the oil. If you have an oil-cooled turbo ('86 and older models, I think), you should practice ardent procedures to allow the turbo bearing to cool down before shutting the engine off. If a water-cooled turbo is installed, cool down is probably not necessary under normal conditions. As a guide, if you are blasting down the I-whatever at 85 mph and quickly come in for a landing to fuel up, it would be prudent to allow the engine to idle for a period of time to allow the turbo to spin down and the bearing to cool down. An old Saab racer once told me that after hard driving, one should allow it to idle 'til the cooling fans switch on. That was 20 years ago when all Saab turbos were oil-cooled, but it would be wise to allow some cool-down for modern units under these conditions.
The turbo can spin at outlandish RPM's, somewhere around 150k, I've been told. At this rate they're generating a great amount of heat, and as the oil flows through the turbo mechanism it is obviously being exposed to these frying-pan-like temps. If it is very hot and the engine is shut down, some oil is obviously stranded at the bearing and subjected to abnormally high temperatures. Fossil oil isn't nearly as resilient to these extreme temps as synthetic, and can coke (solidify) under the heat in a worse=case scenario, and otherwise break down by oxidation rapidly at best.
When I had oil-cooled turbos, and to a lesser extent with newer units, I just tried to drive moderately before arriving at my destination, and I never do a formal cool-down procedure (idling at rest) unless I've been driving aggresively just before arrival. It's a subjective thing, with no hard-and-fast rules--as you just establish a rapport with the mechanicals of the car. I try to imagine how hot the turbo is in that particular situation and react accordingly.
>> I have a '97 CSE with fossil in it. The local Saab dealer says that I shouldn't use synth because I drive for only short distances and the amount of condensation that will form in the crankcase between my 3 month change interval makes spending the extra $ for synth an unrealistic option. I want to maintain this car well for as long as I own it and am willing to go to synth if I should.
Hmmm, I've never heard this rationale before, but there is some truth to what he says. However, I disagree with the idea that you shouldn't use synthetic oil under these conditions. How do you define "short trips"? A "short trip" is any trip that doesn't allow the oil to reach near operating temperature, or high enough to burn the water and other undesirable byproducts of combustion out. If it is allowed to remain in the crankcase, sulfuric acids can form from the water and sulfur in the gas that do no good for anything. The service manual for my '66 Corvette has a boldfaced warning in the forward about "crankcase dilution," and how it can cause pitting of cylinders and other damage.
The oil has additives to absorb these acids and other contaminants, to a point. What your dealer is essentially saying is that the TBN (total base number, a measure of the oil's ability to absorb acids) will approach zero and will become exhausted quicker if you do frequent trips that don't allow the oil to heat up sufficiently. At the point TBN approaches two or three, the oil should obviously be changed. The oil will exhaust its acid-absorbing capabilities faster under these conditions, so it should be changed more often, obviously a more costly proposition with synthetic oil than fossil.
Now, again, how do you define "short trips," and how often? I can't imagine a good synthetic needing replacement before 5k miles, though a fossil should nearly always be changed at 3k intervals or less, especially on a turbo motor. Many, including me, believe a turbo motor should never see fossil oil. Do the math: If you drive 3k miles on your $1.50/qt. fossil oil or 5k miles on your $4/qt. synthetic, how much poorer are you going to be by using the better oil? $20/year maybe? $30?
I change oil at 10k intervals because I lab-tested it twice (at 5k and 10k) and determined the latter to be best for me. At 10k my TBN was about three, as I recall, which is rather low--as it starts out at 8 or 10 when first installed. I don't do many short trips, and in fact go out of my way not to allow the engine to shut down 'til the oil is at least 120°F.
Going to at least 180° is best.
posted by 68.11.251...
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