3 Questions About Synthetic Motor Oil Change Intervals

by admin on April 7, 2010

I received these 3 questions about synthetic motor oil change intervals today from a friendly subscriber.  They are great questions because if you are not careful you can end up spending way too much money paying the greedy oil companies or at the other end of the spectrum you can end up with engine damage, reduced performance, burning oil etc…

So here are the questions:

#1 I know the 3,000 mile oil change interval is a myth for conventional motor oil but how about synthetic motor oil?

#2 When is the right time to change my oil if I have not met the mileage limit of the oil yet?

#3 If I am using oil analysis is an initial higher TBN number better?

Now those are some great questions and here are some answers:

#1 The line between synthetic and conventional oil were blurred back in 1997 when Castrol was able to get a court ruling allowing group III basestocks to be called “synthetic”.  So as far as the 3 month 3000 mile myth goes I can’t make a clear distinction between conventional oil and synthetic oil.  I can say generally that the 3K myth is more so a myth with synthetic.  “Good” synthetic oils with a decent additive package should out last a traditional conventional oil.

#2 and #3 have a common issue of TBN.  It is important that the detergents and alkalinity (related to TBN) of the oil are provided for an anticipated duration of the oil in the engine. Motor oil additive packages are designed differently for extended change oils.  Typically the TBN would be higher to make sure the engine environment does not become acidic and corrosive.

Very short drives are hard on oil (like down to the grocery store).  Typical stop and go driving in the city is considered severe conditions with respect to oil change intervals.  If the engine is not run and allowed to really heat up and bake out the oil then unburnt fuel and moisture dilute the oil and break it down.

For that reason it is best to start up and run down the highway for greater than say 15 minutes to really allow the engine to cook the oil “dry”.

Bottom line is if the oil is not designed for longer life with higher TBN more detergents etc then it will deplete its useful life sooner depending on your driving habits.

Additive packages in oils are very complex.  Every additive comes with a benefit but also an associated cost and each additive has to be balanced with the other additives.  It takes a lot of testing to get it right.

The best way to evaluate the longevity of a motor oil is run it in your engine and perform periodic oil analysis.  If the additives are balanced properly I would expect the TBN should stay elevated longer therefore protecting your engine from corrosion for longer drain intervals.

I like the extended performance Mobil 1 Synthetic oil and the AMSOIL Synthetic Oil.  I have been using AMSOIL with oil change intervals of 1 year for about the last 7 years on the same two vehicle with good results.

On your last note I agree it really is difficult to feel a 2% change in horsepower or detect a small percentage improvement in gas mileage.  A brake caliper dragging a little bit or low tire pressure makes that much of a difference.  That is why I am always skeptical of people that say they could feel a power improvement when they changed their oil or they added some moly additive to their oil and they got better gas mileage etc…

If you want you can read the revealing report on how to prevent an engine catastrophe by going here (it compares Mobil 1 Synthetic Motor Oil and AMSOIL and etc….)


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