[T3] Stupid question #395 Oil change time?

[Jim Adney]

I think there are a few basic rules we can apply to oil changes.

1) Change it every 3000 miles, or at least once a year.

	a) Short trips are harder on an engine, but short means too short for the 
	engine to fully warm up.

	b) Warmup happens faster in warmer weather. 50 F may seem cool to 
	you, but that's nothing compared to -20 F.

	c) If you don't have the complete thermostat, linkage, and flaps, 
	installed, you should assume that your engine NEVER warms up.

2) If this car is going into storage for an extended time, change the oil first, 
so the engine sits with fresh oil, not contaminated oil.

3) Zinc, in the form of ZDDP, used to be an essential part of any engine oil, 
but it poisons catylitic converters, so it's been dropped. Cars with Cat Cons 
switched to non-ZDDP oils and roller cams, so now there are few standard 
oils that are good for flat tappet engines. I THINK that any oil out there that 
has the API seal on the front lacks ZDDP. You might find diesel oils that lack 
this seal. (This API seal has a zig-zag border and is always found on the 
front of the container, not to be confused with the smooth round Service 
Rating on the back.)

	a) ZDDP is especially important during breakin of the cam. It may not 
	matter so much in a well broken-in engine.

	b) It sounds like GM's EOS is a good source for ZDDP.

	c) I'd be surprised if STP had ZDDP in it, as that would make it a very 
	bad choice for any modern engines. I'll check on that later this week at 
	my FLAPS.

4) Going to higher viscosity oils, like 20W-50 may seem like a good choice, 
but that can be misleading. It takes more engine power to pump a high 
viscosity oil and it may take longer for the higher viscosity oil to reach critical 
places in the engine upon startup. High viscosity oil is a poor choice for 
engines that spend a lot of time in warmup. This is not as simple a problem 
as it might seem.

I used 20W-50 only one time, when I had an oil leak. That slowed the leak 
until I had a chance to fix it. My favorite oil would be 20W-40, but I haven't 
seen that available for decades. I'm now using 15W-40 diesel oil that lacks 
the API seal.


I've never fogged my engines. In fact, I had to google it to find out what this 
meant. Seems like it's especially popular in the 2-cycle world, but it makes 
some sense. Anyone else out there do it?

Quiz Question #1, for extra points: What does the W in 20W-50 stand for?

Quiz Question #2, for extra points: would 30W-30 have constant viscosity?

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