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Motor Oil is Hydraulic Fluid

Written by Ed Sanow

Motor Oil Is Hydraulic Fluid

All of today’s police engines (all of them) have a variable valvetrain or lifter-controlled cylinder deactivation. The Charger’s 5.7L V8 and the Tahoe’s 5.3L V8 have both. All these engines depend on the motor oil to act like hydraulic fluid…and the two are not the same!

All of this places great importance on the oil. Motor oil has always been responsible for 100 percent of the engine lubrication and 40 percent of the engine cooling. Variable valvetrain systems place a third demand on motor oil: It must now also act as a hydraulic fluid and it is now responsible for 100 percent of that hydraulic action.

On any engine with a variable valvetrain or cylinder deactivation, the wrong weight of motor oil, or dirty oil that has consumed its anti-foam additives, can throw a Diagnostic Trouble Code or Malfunction Indicator Light. That is because the hydraulic oil properties are not what the powertrain control module (PCM) expects them to be.

The PCM expects the variable valvetrain to respond in a certain way. If the oil has mini-bubbles in it, or is too thick or too thin, the valvetrain will not do what the PCM tells it to do or do it exactly when the PCM tells it to do it. The PCM will sense trouble…and let you know about it.

 

Synthetic Oil and New Oil Standards

The advantages of synthetic oil in police work are clear. Synthetic oil offers better engine protection during a cold start, when 60 percent of engine wear takes place. Synthetic oil also offers better protection in extreme cold, extreme heat, and during periods of extended idling…you know, routine police work.

Synthetic oil increases the oil change interval from 3K or 5K miles to between 10K and 12K miles. Yes, synthetic oil costs more than conventional petroleum oil. However, with this extended drain interval, synthetic oil is actually a cost savings of around $110 per vehicle per year.

Get up to speed on synthetic oil because most 2011 and newer GM vehicles require the use of dexos™ spec motor oil. The only motor oil that can meet the new GM dexos™ oil specification is synthetic oil. In some cases, a full-synthetic motor oil already met the dexos™ spec. Think Mobil 1. In other cases, a tiny amount of expensive molybdenum had to be added for increased anti-wear control to meet the spec. This element may be in addition to, or in place of zinc, which is the most common anti-wear compound.

The GM dexos™ spec was merely a year or so ahead of the oil industry’s new GF-5 oil spec. GF-5 will replace GF-4. These specs are developed by the International Lubricants Standardization and Approval Committee (ILSAC) with input from automakers, oil refiners and additive makers. For those more familiar with the American Petroleum Institute (API) rating system, the new ILSAC GF-5 will carry an API “SN” label.

The industry-wide move to synthetic oil was coming anyhow. GM dexos™ just got the jump on the rest of the industry. The new GM dexos™, ILSAC GF-5 and API SN oils are superior in almost every way to conventional, non-synthetic motor oil.

 

No Air in Hydraulic Fluid

That said, the GM dexos™ oil has one specification that ILSAC and API labeled oils may not have. This important dexos™ requirement is for better resistance to aeration, which is the whipping of air bubbles into the oil. Engines with variable valvetrains (variable camshaft timing) use engine oil as a hydraulic fluid to change the timing on camshafts or turn on and off hydraulic lifters in cylinder deactivation engines.

If air bubbles are in the oil, the lifters will not act as fast as the engine controllers expect. The hydraulic pressure from the PCM signal will have to compress and crush the air bubbles first before the proper hydraulic force can be applied. This causes enough of a time delay to cause a slow response or an under-reaction. This is exactly like air bubbles in brake fluid that cause a spongy brake pedal.

On the mild end, oil with whipped up micro-bubbles can limit engine performance and fuel economy. On the extreme end, this slow response can throw a MIL or DTC check engine code. The wrong weight of oil can cause exactly the same kind of improper response to the PCM signals, either too fast (for lighter weight oil) or too slow (for heavier weight oil).

For all practical purposes, put Mobil 1 or a similar dexos™ compliant full synthetic oil in all police vehicles. The use of synthetic oil has always been a good match for how we use our vehicles. Oil change intervals simply must be followed on any engine with a variable valvetrain or cylinder deactivation. The right oil to use is embossed on the filler cap, both weight and blend (petroleum, synthetic blend, and full synthetic). Use the right oil. Change it on time.


Published in Police Fleet Manager, Mar/Apr 2013

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