[T3] 73 T-3 oil temp?

[Jim Adney]

There's probably no one among us who has not been down the "more oil 
pressure is good" road. Good oil pressure is the ONLY thing a stock engine 
can tell us, but it's important to keep in mind that VW thought any oil 
pressure above the oil pressure switch trip point was okay. That trip point is 
something below 10 psi.

The reality is that the pressure we measure depends on the flow and the 
resistance to flow beyond the point where pressure is measured. So, yes, 
tight bearing clearances will increase pressure, but so would smaller oil 
passages.

The degree of lubrication we're getting in our bearings has almost nothing to 
do with the oil pressure we are measuring. The oil pressure we measure is 
just an indicator that we are probably getting oil to the bearings. Keep in 
mind that if there was a complete blockage of the oil passages after the 
measuring point, we would measure huge pressures, but that would not be a 
good sign.

The important thing to understand is what oil does after it's delivered to the 
bearing. Think about a crankshaft that's perfectly centered in its bearing. 
That crankshaft does not need ANY lubrication, but once it starts to be 
pushed off center, that's when we want oil in there.

The designers of the engine know the directions where the the crankshaft is 
going to be pushed, so they design the bearings so that oil will be delivered 
elsewhere. These will be low pressure areas around the crank.

When the crankshaft spins, it drags the oil around and pulls it into the area 
where clearances are tight. The rotation of the crank pumps the oil into the 
places where it's most needed, and this pumping produces pressures, 
WHERE IT'S NEEDED, that can be HUGE. These pressures dwarf the 
pressures we have been measuring in the delivery system. I don't know 
actual numbers, but I'm willing to guess that they are hundreds or thousands 
of psi. This is the hydrodynamic pressure developed in a journal bearing that 
makes it so effective.

Going for higher viscosity oil will certainly increase the measured pressure in 
our engines, but this may be misleading, as it is bound to lead to reduced 
flow. It also tends to open the oil cooler bypass valve, meaning that less oil 
passes thru the oil cooler and the result is hotter oil. So thicker oil may well 
be counterproductive.

Thicker oil also requires more energy to pump, and this extra energy ends 
up as heat in the oil. More energy lost in the oil means less energy to the 
wheels and higher temperature oil.

So, all that said, I come back to the fact that while ALL of us have walked 
down the thicker oil is better road, but it's not that simple. Take a hint from 
the engineers who designed and tested our engines over decades. They 
knew the whole story and they put their recommendations in our owners 
manuals. Take a look at yours and see what you can do that sticks to its 
guidance. Yes, keep in mind that we can no longer buy the oil made at that 
time, but we can make thoughtful decisions from what is available today.

For what it's worth, I usually use a cheap Diesel 15W-40, partly because it 
meets older additive standards and partly because it's a multi-grade oil that 
doesn't get as thin as most of the auto stuff available today. Bonus: Diesels 
tend to run dirty, so I figure this is a lot like our filterless engines. For zinc, to 
preserve my cams and lifters, I use a Lucas additive that is very reasonably 
priced.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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