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Re: [T3] too much oil -vs- gas in oil

On 13 Apr 2006 at 19:19, Will McCreight wrote:

> Today I prepared to syphon off the "extra" oil, only to discover that 
> when dead cold, the oil was right--halfway between the marks!!  In 
> retrospect, I could have been checking the engine when it was too warm 
> or something...will write more later if problem unresolved.

While it's certainly possible for the engine to overheat, I don't think this 
ever has an adverse effect on the measured oil level. While hot oil expands and 
takes up more volume, the engine case also expands, which will bring the 
measured level down, so the 2 effects compensate for each other somewhat.  

The only problem that actually occurs is when trying to measure the oil level 
in a cold engine where the cold oil was not given a chance to drain back down 
into the sump. This often leads to an erroneous "low" reading. Hot oil drains 
back to the sump very fast, so this generally gives correct readings.

> Also, Jim Adney, in his usual excellent reply said:
> BTW, I don't recommend heavy weight oils. They DO reduce leaks, but 
> they also restrict flow to the engine and require more HP just to 
> pump around; that's power wasted. It's not really true that the oil 
> has to be thick to lubricate well. What the bearings really need is a 
> plentyful supply of oil, and heavy weight oil can sometimes actually 
> starve a bearing.
> My question then...should I be using strict 30 or 40 weight oil, a 
> multigrade oil (and if so, which weight(s)), or something else?  The big 
> reason I made the switch to a thicker oil had to do with the incessant 
> oil light flicker after installing the VDO dual sensor so I could add an 
> oil pressure gauge.  Is it OK for the pressure to run at perhaps 5 psi 
> at idle, or should I be concerned?  Back when I was just an "idiot" with 
> the light only, I didn't give it a thought, but now with oil 
> pressure/temp/vacuum/voltage/tach (and considering CHT), I am getting a 
> feel for what happens when, relatively speaking.  Making an oil weight 
> choice based on a gauge reading may not be a wise decision...open to 
> opinions.

Straight weight oils are hard to find these days, so multi-grades are just 
about your only choice. My favorite would be something like 20W-40 in the 
summer and 10W-30 in the winter, but I haven't seen those around in the last 
couple decades either. What I end up using is 10W-30 in the winter and 10W-40 
in the summer. 15W-40 would be an even better summer choice, but it tends to be 
very pricey.

My personal opinion is that oil pressure at the gauge is ALMOST irrelevant. The 
only importance I think it has is to show that there is SOME pressure, so that 
you know the pump is still running and that it's not sucking air. The engine's 
oil pressure needs at idle are minimal and that's all the pump will be 
delivering at that point. Given the variation in gauge and sensor calibrations, 
it's not unusual for a hot engine to have an oil light that flickers at idle.

OTOH, a cold engine should not flicker at idle. This has almost nothing to do 
with the expansion of the bearing spaces, but rather the viscosity of the oil 
when cold.

An oil pressure light that flickers on cold idle or flickers at speed (either 
cold or warm) is a warning that there's something wrong or that the engine, or 
oil pump, is just getting excessively worn. An oil light that flickers on a 
hard stop, or going around a corner is a sign that the oil level is low.  

Of course, a new sensor that trips at a higher set point may be the only 
problem that you have.

Jim Adney
Madison, WI 53711-3054

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