Hey guys,

A mechanic was recommending that use 10w-40 diesel oil in my pajero 3.5 v6 gas vehicle after I suffered a bearing knock. I am just replacing the crankshaft and was wondering if the advice being given by the mechanic is a good one. He says with this oil, there is no way the vehicle can develop a bearing knock, even if it is low on oil. He claims all of his vehicles, a honda car, a mazda van and a mitsubishi pickup have been using this same oil for a few years now, with no problems.

Should I take ths chance?

Please advise.

It would take a lot of evidence to convince me that the Mitsu recommended oil does not provide adequate lubrication. That is hard for me to believe. True there are probably better oils but all should be adequate.

With that said, many Porsche enthusists are running diesel oil in their gasoline powered flat sixes. The data shows that the old formulation (CI) has a ton of ZDDP which provides high shear lubrication. This is important when oil breaks down and is your last line of defense. This is true in high performance cars, but a street SUV should not be encountering these conditions. If it does, it probably means that the engine oil has leaked out, oil pressure drops to zero and the driver still drives the car...

So why isnt there more mechanics and enthusists running diesel? Two reasons IMHO:

1) The latest formulation of diesel motor (API-CJ) is low sulfur content and may not have the high levels of ZDDP as the CI version did. The EPA is mandating that ZDDP is bad for catalytic converters and bad for the environment. So when you buy any diesel motor oil made in the last year, your not getting the added protection your wrench likes.

2) Instead of diesel, some go with boutique brands like Redline ($8-10 / quart) or Brad Penn (almost same price) which have high ZDDP despite the move by big oil to reduce these levels.

Remember what I said - Unless you are running your engine for hours on end at redline or running your motor so hot that the motor oil breaks down, YOU WILL NEVER NEED the high ZDDP protection.

I suggest that you go for a good quality synthetic motor oil. Synthetics have higher HTHS (high temp, high shear) protection over conventional motor oil so it will protect bearings under higher stresses. They are also engineered (like the 0w30) to have better flow in cold temps for better bearing lubrication at start-up.

The big debate is whether syn oils are overkill. Many that run conventional oil get 100s of 1000s of miles out of their motors. I run syn cause it makes me feel better and scientifically speaking it is better than conventional.

Why did you have bearing failure? Did you run low or out of oil? tons of cold start-ups? Was is a problem from the factory? Did you go extra long between changes?

Motor oil is religion for many. Others will have different opinions than myself. Just my 2 cents. Whatever you decide, make sure you check your oil level every 4th trip to the gas station, make sure its not low and change your filter with the oil every 5-8k miles and the bearings will be fine.

We havent even started talking about transmissions yet. That is the most neglected mechanical system on your truck. Change its fluid too!

Thanks much Tj90, for this piece. From what you have asid I am more convinced to use the fully synthethic oil on my vehicle. As for the transmission, I am also thinking of giving it a full flush of its system so hopefully I can get less stress down the line.

Also keep in mind that diesel motor oils have significantly higher detergent concentrations than conventional gas motor oil, or even synthetic. In a higher mileage vehicle where they could deposits of gunk in the oil pathways, diesel motor oil may loosen those deposits, causing blockages.

For this reason I would be cautious about switching to diesel motor oil. If you wish to, you might want to make the change gradually, mixing with conventional motor oil over several oil changes.

Very well stated TJ90 , very well. Also ilovetrains. I have seen wut yu are explaining. I have seen people put diesel oil in their petrol Pajero's and Lancruiser Prados , and Hilux and Hilux Surf's and have had a similar issue. Timing noises, of deposits clogging the timing tensioner ports , and the tensioners not working properly. Oil drain backs soon after becoming clogged etc and all sorts of disasters after running diesel oil in their on road vehicles after a while.

I think a wise man will take this advice as there seems to be problems down the road if I switch to the diesel oil in my engine. This is what I am trying to avoid. I guess it will have to be the full synthethic oil for me.

Thanks guys, I really appreciate this advice.

I love this site.

If you go to replace the tranny fluid, differentials or transaxles, make sure your wrench uses only approved lubricants. Many quick change places do not have the correct fluid type/viscosities your truck needs. The safest thing to do is have the dealer change the fluids or buy the Mitsu fluids and take them to your wrench.

I wish I had a dollar for every post Ive read on this board of a mechanic putting the wrong transmission fluid in their truck and now theyve got problem.

My opinion on these fluids is 180 degrees different than motor oil where you have more latitude with brands and viscosities.