I have always been told that engine oil should be changed as determined by time as well as mileage. In the past I have followed this rule in my cars but then there is only four or five quarts of oil, not over six gallons. What is the accepted rule on this, I really don't want to ruin a very good Cummins engine because of this but I would like to hear what others think.

Also, if changed due to age and not mileage does the filter need changing at the same time?

As most on the forum know, I'm a recovering lube engineer.

Time IS an important factor in determining oil change intervals - moreso for RVs than linehaul tractors. But there's more to it than just the calendar. Note that oil should be discarded when the additive package is depleted (which is why synthetics don't justify the cost in diesels).

The real culprit in oil deterioration is water. Yeah, impurities in the base stock and in fuel contribute, but most can't cause issues without dihydrogen monoxide.

Since the major nasty is sulfur in fuel introduced into the crankcase by tiny blowby, ULSD has lowered its effect markedly. But with water present, sulfur becomes (over time) sulfuric acid. The afore-mentioned additives coat the metal and protect it, and also coat the H2SO4 droplets (and in some oils, chemically neutralize them).

The bottom line is to keep moisture out of the crankcase, but this is impossible. Now motor oil is formulated to hold moisture in suspension, where it is relatively harmless. The remedy is to boil off the water.

So, drive the coach at least 10 miles (20 is better) once a month or 6 weeks, and you need be concerned about changing the oil only once a year.

This applies only to oils I'm familiar with: Shell Rotella T, Mobil Delvac, and Kendall Super DIII.

They really should ban that nasty dihydrogen monoxide! Major component of acid rain, tumors, causes many deaths by accidental inhalation, solid form causes tissue damage to outdoorsman after skin contact.

Make sure to cook it out of your motor oil regularly if possible.

I had a poster in my chem classroom for years, such nasty stuff!

quote:

They really should ban that nasty dihydrogen monoxide! Major component of acid rain, tumors, causes many deaths by accidental inhalation, solid form causes tissue damage to outdoorsman after skin contact.

Yeah! yeesh! and some people even drink that crap.

"I never drink water - fish fornicate in it..." - W. C. Fields

I once got the tongue lashing of my life from an old Scotch drinker when I put "Coleman Young's Detroit city water" in his Scotch. He said the same thing only I think he used a different f word...