[T3] Stupid question #395 Oil change time?

[Jim Adney]

Some comments on oil. Some of these are pet peeves, so forgive me if this 
seems pedantic.

1) There's no such thing as 30W oil. Same for 40W, 30 w, 30w, 30 W, 40 W, 
40w or 40 w. Likewise for 10, 20, and 50. These are properly called SAE 30, 
etc.

The W stands for Winter and only appears on multi-viscosity oils, as in 
10W-40, etc, or SAE 10W-40 to be complete.

I note that the Tractor Supply web site makes this mistake consistently: 
Although the labels on their bottles all state SAE 30, etc, Tractor Supply 
consistently describes them as 30W or 30w. This is simply a 
misunderstanding of what the W means, and is wrong.

It's common for people to mention 30-weight oil and this often gets 
condensed to 30W, because people see that W on the label and confuse the 
W with weight. I accept this use of "weight" even though there's little 
difference in mass and what we mean is viscosity. What we mean, and what 
we should recognize, is that we're talking about SAE 30, SAE 10W-40, etc. 
(SAE = Society of Automotive Engineers, the group that wrote the 
specification.)

In multi-vicosity oils, the W (winter) viscosity is measured at (I believe) 32 F 
(0 C) while the hot viscosity is measured (I believe) at 212 F (100 C.) But the 
2 measurements are made on different viscosity scales, so this can be 
misleading. A 30W-30, if such a thing existed, would not have the same 
viscosity at both 32 F and 212 F. SAE 20W-50 is thinner at 100 F than it is 
at 32 F, but it hasn't thinned down as much as SAE 20 would have.

Note that this means that SAE 20W-50 does NOT get more viscous as it 
gets warm.

I believe the viscosity scales used for gear oils are also different from those 
used for engine oils, and they also use different scales for hot and cold. So 
you'll see 80W-90 gear oils. These come in grades like GL-4 and GL-5, 
where I believe GL stands for Gear Lubricant.

2) I don't see any reason why you could not mix any petro engine oils 
together for some kind of intermediate outcome, but it's quite possible that 
mixing synthetic and petro oils might not work. I don't know about that.

3) I bought a 16 oz. bottle of Lucas "Racing ZDDP / TB Zinc-Plus / Engine 
Break-In Oil Additive" " #0 49807 10063 6 at my FLAPS yesterday for $13.61 
after tax. The Lucas spec sheet lists this as having 43,000 ppm of zinc, so I 
ran the numbers to see what this would mean for us.

We should get 4000+ ppm additional ZDDP if we add 4 oz (1/4 bottle, $3.40) 
to one oil change (5.3 pts = 42.4 oz)

or 2000+ ppm of additional ZDDP if I add 2 oz (1/8 bottle, $1.70) to each oil 
change.

The bottle states that it is NOT recommended for any vehicle later than 1994 
or that requires oil meeting API (American Petroleum Institute) spec SG or 
later. "May cause damage to catalytic converters." I believe the latest spec 
called out for our cars was SD.

I'm going to start using ~2 oz with each oil change. I agree with Brian that 
the fancy, high-priced oils are probably not worth it for us, but I think the 
ZDDP probably is. I would use twice as much for break-in of a new engine.

3) Because it gets quite cold around here, and I have engines that get driven 
occasionally in that cold, I think I'll continue to use the cheap Diesel SAE 
15W-40 year round but with the addition of the Lucas ZDDP additive. This is 
a cheap, effective solution for year round use in climates that see temps well 
below freezing. If I lived in a warm climate, I'd find SAE 30, add 2 oz of the 
Lucas ZDDP additive and use that year round.

4) It may also have been Brian who mentioned how it was more important to 
value flow to the bearings over pressure in the oil passages. I agree with 
this. The only pressure that counts is the pressure that's developed between 
the rotating shaft and the bearing. I'll try to explain why oil pressure isn't as 
important as oil volume.

If you don't already understand this, it may be helpful to draw yourself a 
picture of a rotating circle within another circle. The rotation of the center 
circle drags the oil around it.

Now draw another set of circles with the center circle a bit off-center. This 
creates a "wedge" where the dragged-along oil gets pushed into a tighter 
and tighter space. This is where the rotation of the center circle creates 
pressure, and that's the pressure that keeps the 2 parts from touching. 

The pressure developed depends on the speed of rotation, the viscosity of 
the oil, the width of the bearing, and how tight the wedge gets. It gets higher 
as the shaft gets closer to touching the bearing. If the clearance gets close, 
the pressure can get extremely high: much higher than the oil delivery 
pressure. This is what keeps the two parts from ever actually touching, and 
this is why plain journal bearings work so well.

Note that this all depends on the oil being virtually incompressible, so if 
you're delivering air or foam to the bearing there will be almost no pressure 
developed in the wedge and the shaft is likely to rub against the bearing. 
This may happen each time we start, but there is likely to be a thin layer of 
oil still there to prevent actual metal to metal contact.

Note that this explains why the oil entry points in a bearing may seem to be 
in odd places. The people who design engines have found that they need to 
inject oil at the low pressure areas so that the rotation of the shaft can put 
the most pressure where it's most needed. The worst possible place to try to 
inject oil would be where the wedge effect is trying to build pressure; that 
would just give the pressurized wedge of oil a path to escape, allowing the 
parts to touch.  

This may help you understand why it is not necessarily a good idea to 
choose a higher viscosity oil. The higher viscosity will make it harder to 
pump, especially during warmup, and may be slower getting to the places 
where it's needed. It takes more engine power to pump, so it will cause more 
wear on the pump and the parts that drive the pump. If you have a filter, it 
will be harder to pump thru the filter and that puts even more load on the 
pump, its drive, and the engine. While the higher viscosity will increase the 
wedge pressure in the bearing, that's probably not as useful as being SURE 
of plenty of flow into the bearing.

5) Note that in 1970 VW increased the size of the oil passages, but didn't 
change the size of the oil pump. If that was all they did, this would have 
meant the same flow, but with reduced oil delivery pressure. However, VW 
also changed the pressure relief system, so this muddies the picture. They 
didn't change the oil pressure switch, so it seems likely that the pressure 
relief system changed to keep the oil galley pressures pretty much the same.

However, in 1972, they increased the size of the oil pump without changing 
anything else. This clearly increased flow and should have increased 
pressure, too, but we don't see the pressure increase, probably because the 
pressure relief valve works well and because of where we measure the 
pressure, unless we use oils with higher viscosities than VW ever 
recommended. These late engines seem to have better longevity than the 
earlier ones.  


Thanks for reading. I hope it wasn't too painful.

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