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Author Topic: Why would Onan specify gasoline oil in a diesel generator?  (Read 2725 times)

« on: August 16, 2006, 03:58:00 PM »

I have a 7500 watt Onan diesel RV generator.  It is an older 1993 model and is not the newer quiet model.  The manual specifies gasoline grade oil.  I called Onan today and they said that it was all right to use diesel oil.

Why woould they spec gasoline oil for a diesel engine, particularly a Kubota engine?

Brian Elfert
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"Foolish Pleasure" 35' MC5A

« Reply #1 on: August 16, 2006, 04:19:57 PM »

Brian, just to relieve any consternation you might have about the Kubota powered Onan generators....We were Onan dealers for over 30 years and you can use any of the multi-grade automotive oils of that period in your generator. 

For example, we used Valvoline 15w-40 in all the generators we serviced for most of those years. Now, my Ford Ranger and my 2005 Scion (which specificlly state that 5W-20 should be used in them) but I still use the the 15W-40 in my older vehicles.  Feel free to use a quality brand of 15W-40 in your generator.

Hope that removes any anxiety you might have on the topic.


True friends are difficult to find, hard to leave and impossible to forget.
Clarke Echols
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« Reply #2 on: August 16, 2006, 07:59:20 PM »

Back when I was younger  Grin motor oils came graded MS/DG (motor severe/diesel general) for general
automotive use and for light-duty diesel engines.  We used a diesel-grade oil in or Cat D4 farm tractor,
but bought the stuff in 55-gallon drums.  So all of our cars got the same stuff.  The diesel grades have
more detergent.

Today's oils are different.  They have specialized additives for specific engine types, so you see a bit
more "pickyness" about oil types and grades.  But for light-duty applications such as small gensets,
you're not looking at a 1,000,000-mile engine that can expect to be running for 20,000 hours before
first overhaul like a DD S-50 or S-60, so oil is somewhat less critical, though it still needs to be good

I'm a "Valvoline" guy.  Won't use anything else -- especially Pennsylvania oils like Penzoil and
Quaker State, due to high paraffin content.  Penzoil really deposits an unbelievable amount of
gunk all over the insides of engines where it is used.  I pulled the pan off of a Cadillac 425 engine
after 100,000 miles following a custom remanufacture (I did the assembly, a man I know and
trust did all of the meticulous machine work and it ran like a fine Swiss watch).  The inside of
that engine was unbelievable!  You could have wiped it clean with a paper towel.  That's what
100,000 miles with Valvoline gets you.  Before the reman, the block sat in the caustic tank for
3 days to get rid of the sludge and gunk from the old Penzoil from previous years.

I'm told that Castrol is a good oil brand, and I suspect that Texaco and others are OK too.
My physics professor from the 1960s was a Standard Oil bulk distributor in an earlier life, and
he was quite vivid in explaining the problems encountered with Pennsylvania-based oils, especially
in the high mountain valley of Southern Colorado where I grew up where the winter temperatures
can get to -40 (F or C -- doesn't matter; that's where the two scales cross over each other) and
frozen gas lines are a major concern (I was stranded close to a half-mile from the nearest telephone
one night in December 1961 due to a frozen gas line in my dad's car on my way to take a night-time
final exam at college in Alamosa.  It was 40 below).

Alamosa is frequently the coldest town in the US in the middle of winter.


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« Reply #3 on: August 17, 2006, 07:10:40 AM »

Clarke, funny i have experienced the exact opposite perfomance from oil, i had a caddillac 390 engine that was so gummed up it took a quart of oil less than it did when i got it, valvealine was the only oil ever used in it, i ise penzoil in my cars and have for yrs including one with more than 200 k miles without any maj problems, i have not seen the buildup in the engine you describe, i wont even put valvealine in my lawn mower
Frank Allen

« Reply #4 on: August 17, 2006, 09:43:52 AM »

I generally use whatever is cheap for my oil needs.  Never had a problem yet.  I do use really good synthetic oil that costs $6 a qt for my VW TDI as it has a 10,000 mile OCI and has EGR.

My brother lost an engine at 60k miles due to oil issues, but he bought it used at 50k miles and the mechanics said they didn't think the oil had ever been changed in it before he got it.

Brian Elfert
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