Viscosity Breakdown? 2

[itsmoked]

Have a case where a Duramax pickup pulls an hour long grade on the Nevada/California boarder frequently. It's turbo'd and as normal for its ilk has a plate heat-exchanger for oil cooling bolted to the side of the engine using radiator water for the thermal dump. Oil is synthetic.

What's happening is the normal rock-steady 50PSI oil pressure steadily declines over the hour long climb down to 30PSI. Speed never changes, water temperature never rises, just the oil pressure declines. Once the summit is reached the oil pressure recovers over several minutes back to exactly 50PSI.

Having discussed this with five oil company reps several say 'this isn't viscosity breakdown because the pressure recovers'. I tend to agree. I think this is more along the lines of the oil pressure regulation actually causing the pressure drop because of either adverse thermal response or from a small (normal) viscosity drop.

Oil reps are suggesting the need to blend synthetic oil with non-synthetic at this point.

Thoughts?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

I didn't mean synthetic blends, I meant buying a quart of conventional and blending it with a quart of synthetic. Oil formulations have been changing a lot lately. In the past 3 years there have been 4 different formulations of Chevron Delo 400. It can't be ideal to have an anti-wear package that is half zinc/phosphate based (Delo 400 MG) and one that is half boron based (Delo 400 LE).

 

[SuperSalad]

If you are going to blend a conventional and a synthetic to make your own semi-synthetic, you'd better make sure they use the same additive package so there's no interfering additives.

Like TugboatEng said, when it comes to lubricant additives, half of one type and half of another type isn't likely to result in good performance from either half. Specific levels of specific additives make an oil perform the way it is supposed to.

The only way I could see the reps suggesting that you mix them is if they know the two types they sell use the exact same additive system so it would really only be a base oil change.

Andrew H.

http://www.mototribology.com

 

[itsmoked]

Follow-up.

Well, Chevron XLE 15-40 the synthetic/dinosaur blend was finally acquired. It was a battle as it wasn't apparently for sale yet. It had to be ordered by the western US distributor.

This weekend the oil was changed to the XLE and a run over the Sierra Nevadas on the same exact route, driving the same as always (cruise control 68MPH) up the looooooOOng slope and down the other side.

The results were interesting.

His around town/freeway oil pressure has dropped 5psi from what it was with the prior oil 60 down to 55.
His transmission temperature went up from 175F to engine temp 195F.
His oil pressure on the grade dropped only to 40-45PSI instead of 30-35PSI. Recovering on the downhill just like with the pure dino.
His water temp has stayed rock steady at 195F in all cases.
His radiator fan ran on the grade when it hasn't in the past. (This cost him 1/2MPG.)

It appears this new oil transfers heat better though I'm not clear on how or if that might help it maintain more pressure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SwinnyGG]

It appears this new oil transfers heat better though I'm not clear on how or if that might help it maintain more pressure.

If the oil is shedding more heat, less vicosity change is a possibility, which would maintain higher pressures.

It's also possible that this particular oil just handles the service temperatures better.. temperature/viscosity plots for oils, especially blends, are sometimes very strange, and are MUCh more complicated than the ratings on the bottles make them appear to be.

 

[EdStainless]

Oils have a VI, viscosity index which is the decrease in viscosity as temp increases.
But it isn't that simple, I have sen cases where moving to thinner oil actually lowered temps because of less shear in the oil and better circulation.
In your case the new oil may be maintaining viscosity better, but it may flow better to start with which improves heat transfer, which also keeps the oil a little cooler. The combination would be less thinning under load.

If in the past the fan wasn't running it sort of concerns me about the sizing of the cooler and placement of the sensors. It sounds like the system is working properly now.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube