Diesel 15W-40 oil in an unleaded engine

Tiny
LABHAMSTER2
  • MEMBER
  • 2003 MAZDA 323
  • 1.8L
  • 4 CYL
  • 2WD
  • MANUAL
  • 125,000 MILES
Recently had to add some oil to my car on a road trip, and the only 15W-40 available was a heavy duty diesel oil (Shell Rimula R4 L 15W-40 to be specific). Is it safe to keep using the engine for another week with this oil in it, or should I dump it and change over now?
Saturday, October 10th, 2020 AT 6:53 PM

2 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
No. I would change it out. This oil is clearly not meant for your engine and the longer it is in there, the more potential damage is done to the bearings. Clearly it is not dire at this point unless you used more then 1 quart. How much did you add to the engine?

It is one thing to add a thicker oil but to add an oil for a diesel engine is just not going to help so the longer it is in there the worse it could be. So no matter what, I would just be on the safe side and change it as soon as possible.
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Saturday, October 10th, 2020 AT 6:59 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Hi,

I agree with KaseKenny. A diesel engine oils will have a higher anti-wear design. As a result, different components are added to them in the form of zinc dialkyldithiophosphate. A catalytic converter in diesel system is designed to deal with this added component. A catalytic converter in a gas engine is not designed for it and it can cause damage to it.

As he mentioned, do it at the earliest convenience.

Joe
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Saturday, October 10th, 2020 AT 7:19 PM

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