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Well I thought I did the research but I was wrong. I bought the car at 154k and have done 2 oil changes 157k and 168k with 0W-30 mobil 1 and the car was running great. 48+mpg. I had the TB changed last week 178k and oil changed. (we found out the engine is not original) My mech said I was using the wrong oil and put in 5W-40. Now my mpg is down to 44+. I also have heard that you should not revert to oil after synthetics. Still runs good and is slightly quieter with maybe a little more power. Any suggestions? Or should I just keep going. Thanx Bill
 

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You're supposed to use synthetic oil for diesels, anything that is rated for CI compression ignition. There should be a note on the oil cap and the owner's manual on what oil you can use. The mpg is down due to the thicker oil.
 

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As I read it, I need to stay with 40 weight but I'm going to look for Mobil 1 0W-40 syn. I'm assuming it's OK to go back to syn even though I don't know what was in it before I got the car. Thanks
 

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use of modern synthetics

Well I thought I did the research but I was wrong. I bought the car at 154k and have done 2 oil changes 157k and 168k with 0W-30 mobil 1 and the car was running great. 48+mpg. I had the TB changed last week 178k and oil changed. (we found out the engine is not original) My mech said I was using the wrong oil and put in 5W-40. Now my mpg is down to 44+. I also have heard that you should not revert to oil after synthetics. Still runs good and is slightly quieter with maybe a little more power. Any suggestions? Or should I just keep going. Thanx Bill

A modern synthetic properly formulated will provide much better protection even at a lower viscosity than any petroleum oil. When I say properly formulated I mean that the base stock used should not require a lot of VI (viscosity index) improvers to obtain the stated range. I've been an amsoil dealer since 1983 and have experimented a bit with oil. I've gone back and forth between dino oil and synthetics and back as my budget allowed, gas and Diesel engines. I've even seen a turbo Diesel with a bad turbo seal go from burning a quart in 300 miles to 700 miles when I used amsoil in it...

THE main issue with synthetics is switching TO THEM, not going back to dino oil... Synthetics
will clean an engine... this could clog up an oil gallery in really severe cases, or just fill up the filter early, if you switch, watch the oil and swap out the filter. If you go from synthetic back to dino I can't think of any issues what so ever, except for reduced fuel economy because you've put something in that has higher friction.

I'm not familiar with mobil 1 but if it's a 100% synthetic and not the reformulated hydrocarbons of the '80s, then even at a 0-30W rating it should be fine in our engines and provide you with excellent economy. The key issue at higher temps is the shear strength of the oil. There are a variety of specs you could look at. But even if you lived in Arizona and were running 0-30 mobil 1 in the summer it would still provide better protection than the 5-40 dino oil. Of course I would recommend the amsoil product because I know it would be fine. But any modern synthetic will have those characteristics... unless they've gone cheap with the base stock and use Vi improvers to extend the range... but, if they did that only impacts the lower reading... so your 0-30 would likely just be 30W when the Viscosity index improvers wore out. That's why you have to change your dino oil, the additive package goes away. That's why amsoil can do 25,000 or more mile oil changes. They don't have to use a lot of VI improvers to provide the temperature range. It doesn't wear out, it holds dirt in suspension and the filters can remove it...

One other thought, one of the other posters mentioned having a Diesel rated oil, CI rated, compression ignition, vs. SI, or Spark Ignition. So if it has an API C-rating that matches what
the manufacturer calls out, like CE or CF, and the oil is that or higher, you are covered and it can not void your warranty.

I suspect you misunderstood your mechanic... he must have meant going from dino to synthetic, if he meant it the other way I'd like to understand his logic and reasoning... because in over 40 years of wrenching I've never heard that one. :)

one other note, never break an engine in with synthetic oil... because it won't. :) wait a couple thousand miles first... if you are trying to use an extended drain interval, take an oil sample and have it analyzed... I went over two years and over 17,000 miles on my '91 300D and the oil analysis came back all A's with one B, on a 2.5L TD with over 266,000 miles on it driven hard in seattle traffic.
 
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