Lets talk oil viscosity!

[challenger440]

So the manual for my 73 8v71 calls for straight 30w. oil.  I'm sure that there is 15w40 in the engine now.  Pretty sure that the po has used it (15w40) for quite some time.  Now the question. 

Is multi weight oil really bad for these engines?

  My feeling is that today's 15w40 is vastly superior to the specifications of the 30w from 40 years ago.

I see that many here recommend going with the 30 only.  What are the arguments for and against going with the multi weight? 

   The advantages of the multi weight seems to be, easier starting, higher viscosity at temperature, easier to find on the shelf. 
   The higher viscosity at temperature on a 150,000 mile engine would seem to be an advantage rather than a disadvantage. 

What do you all think?  JM

[Chopper Scott]

All I know is what I have read here and what seems to be common knowledge at oil venders. The DD 2 strokes are supposed to run single weight only. The reasoning is because of the 2 strokes per firing versus 4 on a 4 stroke. The crank and bearings never get a break from the constant pressure and multiweight oils will not handle the load. Synthetic however is ok as it has better properties for such.

[Timkar]

Have only used 40W in my two stroke engines as per manufacturers recommendations.
Lots of info on this site for your viewing pleasure.....

http://www.tejascoach.com/ddcoil.html

[bevans6]

Spend some quality time with the search engine, you'll find your answers.  it's been fought to death.  Two schools of thought, both probably equally valid.  

One is that Detroit Diesel specifically recommends not using multiweight oils because the operating environment inside the two stroke engine (no cooling stroke for the pistons, basically) degrades the viscosity modifiers and the multiweight oil becomes a single weight something fairly rapidly.   Up until recently the ash content of multiweight oils was too high causing the piston rings to coke up.  Basically, DD says it's bad.  I researched this thoroughly and decided it was probably still not recommended if a high quality single weight CF-2 oil was available.

The other is that the US military and all sorts of other people run DD two strokes exclusively on multiweight oil and have for years.  Plus, modern oils all are low in ash content, and the chemistry of the viscosity multipliers has improved tremendously.  DD doesn't care any more so they aren't going to issue new guidelines, and CF2 as an oil type was outmoded as of last year, API cancelled it.  the thought may be said that Military rebuilds things all the time, and that the oil change cycle is probably a lot more stringent than in commercial use.

30 weight is for cold  temperature use any way, 40 weight is for "normal" temperate zone use and 50 weight is for  hot climate use.

Bottom line - use what makes you happy, it's your engine!  Believe DD, believe the US Military, believe the trusted pundit of your choice on the internet boards, believe the guy who sells you oil, believe the guy who rebuilt your engine, believe the guy who secretly wants your engine to break so he can rebuild it and send his kid to college for a few more years.   You get the picture...

FWIW and in case you decide to believe the guy on the internet with no particular qualifications whatsoever - I use CF2 SAE 40 from whoever has it on the shelf, either Walmart or Napa.

Brian

[challenger440]

Interesting.  Now I'm leaning towards 40w cf2.  I would imagine PO used multi because we live in a cold climate (Montana).  +4f and falling as I type.   The ash content is very interesting.  Now should we be worried about the loss of zinc?  Flat tappet motors are suffering very short cam life from the epa taking out these additives.

JM

[RJ]

JM -

Altho this is an older thread, it's worth reading.

http://www.busconversions.com/bbs/index.php?topic=3162.0;topicseen


Part way through that thread, there's a post of mine talking about the differences between multi- and straight weight oils.  That one post alone might help you understand.

As for obtaining the correct oil, contact your local petroleum distributor, you won't find it on the local auto parts shelf.

FWIW & HTH. . .

[bevans6]

There is no loss of zinc or other extreme pressure additives in diesel oil.  If you used diesel oil in your flat tappet gas engines, you'd be fine.  Comp cams and Elgin Cams both recommend diesel oil, actually, for their flat tappet cams in race engines.

Brian

[challenger440]

Thanks everyone.  Brian, that's interesting about the cam manufacturers.  Happy to hear that the zinc is still in the diesel oil. jm

[Iceni John]

If you used diesel oil in your flat tappet gas engines, you'd be fine.  Comp cams and Elgin Cams both recommend diesel oil, actually, for their flat tappet cams in race engines.

What is the difference between S-rated oil (for gasoline engines) and C-rated oil (for diesels)?   When I got my bus there was S-rated straight 40 in it, but I had it changed to CF-2 straight 40 afterwards.   I thought the engine sounded quieter at idle with CF-2 in it, and it definitely smokes less with the right oil.   However, both were straight 40.   What's worse  -  S-rated 40, or CF-2 multiweight?

Thanks, John

[artvonne]

  There appears to be more than enough evidence to conclude using only straight wt oil. If anyone tries to tell you different just move on. The DD two stroke is one of the only heavy truck or automotive engines with transfer ports in the liner that the rings pass back and forth over. It doesnt take rocket science to see that that area creates a very high shear condition, think scissors here. A Multi-vis oil molecules could be sheared in short order in that area. There was a diesel engine test run by MB some years ago which showed running a straight 30 oil gave the least wear against any multi weight.

  By reading the many threads here, it seems these motors generally use somewhere around a gallon per 1000 miles. Nothing at all wrong with having three or four gallons along when you leave home, so your not caught short in Podunk pouring in the wrong stuff.

  Just because a shop uses something particular and claims its not been a problem, is no reason to conclude they are any authority. Motors are tough, and as long as they have any kind of oil they will run quite a while. Its that long term mileage we want to see out of them where running the correct oil pays off. Some outfits go through equipment like we go through socks, and they may never have anything around long enough to see long term trends. Having a single shop owner going against the opinion of many many Bus mechanics and owners and DD's own specs and recommendations, just shows the man to be a rummy. If you follow his worthless advice and lose, he still wins. Educating yourself on this will prevent you from being talked into something you dont want.
 

[JohnEd]

Logistics is a serious matter.  I can well imagine that a cost to benefit analysis proved that the more frequent overhauls were off set by the reduced logistic burden.  Another thing that pops to mind is just what percentage of mil hardware makes it to the suggested overhaul point of wear and what fail simply from malfunctions that are tantamount to acts of God...such as run the thing out of oil or ran it without the air cleaner or over heated it or ?  You know...those things over which we have no control.  Remember that old saw about how to run and treat your DD 2 stroke?  Drive it like you stole it....in the military that would translate to what?

I spent 8 years in the mil but that was the Air Force and we were careful with stuff.

John

[TomC]

Think of 15W-40 oil as a 15 weight oil with polymers in it to make the engine think it is a 40 weight oil.  As previously stated, 2 stroke engines never relax during power, so the polymers squish down and then the oil becomes a straight 15 weight oil.  Straight 40 weight is always a 40 weight no matter what.  I know it is a pain to obtain 40 weight, but so is a premature engine failure a pain in the wallet.

As to the military using 15W-40 in their 2 stroke engines.  The military also only has a 2000 hour expectation between overhauls.  That can be as low as 80,000 miles!  I had both a 6V-92TA and 8V-92TA in trucks and always used straight 40 weight.  Both engines went 500,000 miles before overhaul.  Good Luck, TomC

[luvrbus]

The great oil debate never ends does it you guys would be shocked if you knew what oil Don Fairchild runs in personal 8v92 DDEC engine lol but I am not going to tell Don


good luck

[Lin]

Clifford, I think Don told me he uses Exxon XD 3 40 weight.  I could have it wrong.  Don--you out there?

[luvrbus]

Not now Lin lol that is what he sells you guys or did till Exxon dropped it you buy it as Mobil Delvac now days



good luck