Engine oil questions?

Kirk W · January 24, 2018

A friend who knows that I have both gas and diesel vehicles, just asked me two very valid questions that I'm not quite sure how to answer about engines and oil.

1) Why does a diesel engine require special oil?

2) What is the difference in the oil for gas versus for diesel?

I have known that they require different oils for years and have a few ideas of the reasons, but don't consider myself qualified to explain. Your thoughts?

Pat & Pete · January 24, 2018

Simple answer : Oil meant for diesel motors has a lot more zinc to combat wear than oil meant for gasoline motors .

There are other less important differences , but ...

beemergary · January 24, 2018

Can't go wrong with diesel oil. I race cars and always use diesel for a break in oil. Diesels have solid lifter cams and the zinc has a wear property. My Peterbilt with a Caterpillar 3406b close to a Million miles only has new bearings put in and there was nothing wrong with old. Lots go way over a million milles and just get rebuilt. Non diesel brand oil is getting harder to find. The lead in gasoline had a wear property and they took it out.

mptjelgin · January 24, 2018

A decent explanation here:

http://www.machinerylubrication.com/Read/28576/comparing-gasoline-diesel-engine-oils-

Kirk W · January 25, 2018

3 hours ago, mptjelgin said:

A decent explanation here:

Thank you! That is exactly what I was looking for. Much more in-depth and yet understandable than anything I had previously read. I'll share it with the person who asked me.

fly2low · January 25, 2018

Read the article, and like many things there is a "but"

I used diesel oil in my race car all of the time, and in my turbo Porsche as well. Most tuners recommend a diesel oil because of the increased ZDDP. If you wanted to have some fun, just go to one of their forums, and ask which brand to use. They all agreed on what kind of oil, but which brand was like watching a religious argument

noteven · January 27, 2018

Remember to use the specified viscosity for your engine’s starting temperature and operating conditions.

masterdrago · March 12, 2018

Oil around my house has gotten a little crazy. 10w-40 Pennzoil dino in the Yamaha boat motor, 30w dino in the Kohler zero turn mower, Valvoline 5w-40 Premium Blue Extreme Synthetic Diesel Engine Oil in the Cummins 6.7, and Pennzoil 5w-30 Platinum Full synthetic for the 2001 Towncar. I recently switched to Shell Rotella 15w-40 dino in the mower and have seen much greater oil life when draining (not black and dirty looking). So other than cost, is there any reason to not use Valvoline 5w-40 Premium Blue Extreme Synthetic Diesel Engine Oil in everything? The Towncar calls for 5w-20. Not sure why 5w-40 would not work. It covers the range of viscosity.

masterdrago · March 12, 2018

So I have now read the link info provided above and see how oil might be detrimental if it is designed for a diesel but used in a gasser.

GeorgiaHybrid · March 12, 2018

9 hours ago, masterdrago said:

Oil around my house has gotten a little crazy. 10w-40 Pennzoil dino in the Yamaha boat motor, 30w dino in the Kohler zero turn mower, Valvoline 5w-40 Premium Blue Extreme Synthetic Diesel Engine Oil in the Cummins 6.7, and Pennzoil 5w-30 Platinum Full synthetic for the 2001 Towncar. I recently switched to Shell Rotella 15w-40 dino in the mower and have seen much greater oil life when draining (not black and dirty looking). So other than cost, is there any reason to not use Valvoline 5w-40 Premium Blue Extreme Synthetic Diesel Engine Oil in everything? The Towncar calls for 5w-20. Not sure why 5w-40 would not work. It covers the range of viscosity.

Your oil is supposed to get black and dirty looking. That means it is doing one of the primary jobs that it is designed to do, keeping combustion soot and other residue in suspension until they removed by the filter or by changing the oil. If the acids, soot and particles created by combustion are not removed, they will turn into sludge inside the engine and will cause a lot of issues by restricting oil flow. The reason that detergents are added to modern oils is for that very reason, to keep the inside of the engine clean.

BlueLghtning · March 12, 2018

One of the things we run into in the motorcycle world in the last 10+ years, is a lot oil designed for gas vehicles has friction modifiers to help with fuel mileage, but many motorcycles have wet clutches and friction modifiers can cause our clutch to slip. The other issue is the engine and transmission share engine oil and the transmission puts a lot of sheer forces on the oil. So Rotella 15w-40 or 5w-40 has always been very popular to use in our motorcycles as being primarily a diesel oil, it doesn't have the friction modifiers and it withstands the much harsher treatment our transmissions put on it.

mptjelgin · March 12, 2018

16 hours ago, masterdrago said:

The Towncar calls for 5w-20. Not sure why 5w-40 would not work. It covers the range of viscosity.

I've driven a Ford V-10 since 1999, first with my 1999 F-250 and now with a 2010 F-350. Ford raised some eyebrows when they spec'd 5W-20 for a motor that folks were working hard, but the explanation was that these newer motors were built to very tight tolerances and needed the thinner viscosity to flow adequately.

Since that time I think that Ford has ok'd 5W-30 for the motors, but I've used their 5W-20 Synthetic Blend exclusively and gotten over 160K miles out of the 1999 before I sold it with zero engine problems and now have 135K on the 2010, also with zero engine issues. I would be hesitant to go to a 5W-40 in motors designed for 5W-20.

On edit - I know that your Towmcar doesn't have a V-10, but all of the modular motors in that series spec'd the same oil as I recall.

masterdrago · March 13, 2018

Thanks Mark. I sent a note over to Tom Shalin at Amsoil and he pretty much confirmed what you said. 5w-30 would be fine. I was just looking for a way to have one oil sitting around the garage. I read through the link at machinerylubrication.com and better understand why the newer motors need either C or S oil depending on what engine they run. I'll most likely stick with the dino oil in 15w-40 for the Kohler mower. Other than that I plan on using only synthetics. B4, I mostly thought oil was oil. In the old, old days, 30w for everything.