Can I mix synthetic with conventional oil?

I normally use synthetic (AGIP Sint) oil in my 86 300 SDL. Since the oil light came on and I have a 3 hour drive to do, can I add a quart of 15W-40 diesel oil until I can have the oil changed? Thanks,
-- Jeff
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Yes, no problem.
Huw
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Absolutely. What do you think "semi-syntheitc" oils are?
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wrote:

You guys are nuts -
Most synthetic oils have few if any detergents in them. If he has been using non-detergent oils and now switches to conventional oil, that should be fun
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wrote:

I think, no, I know that you do not know what you are talking about. If an oil meets API or better standards then it has a specified minimum detergency. In a diesel engine of that age then anything with API CE or better will be more than adequate whether it is mineral or synthetic. Any petrol engine built before '98 will be great with API SL and both diesel and petrol engines built since then need high detergent synthetic oil that meets mb229.1 standards.
Huw
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I called Mobil (1-800-ASK-MOBIL) and a seemingly well-informed fellow named Leo there said that:
1. Synthetics can be mixed with conventional oils of the SAME API specification. NO problem at all.
2. Synthetics will be consumed faster if an engine has a blow-by problem or a leak because synthetics flow easier. However, there is no evidence to support the urban legend that synthetics will cause new leaks or make existing leaks worse.
3. Synthetics flow better, flow more evenly with temperature, resist burning better, and maintain their viscosity MUCH better than conventional oils do.
4. There is no advantage to running heavier weight than necessary. Temperature is the ONLY significant factor in choosing a weight. Severe loading (racing, off-road use, towing) is only a factor because of the increased internal temperature caused by the loading. Using too heavy a weight won't protect parts better but it will reduce mileage.
I am a believer in synthetics for the following reasons from my own personal experience:
1. I filled two '92 Ford Festivas, and '92 and a '95 Escort from new with Mobil 1 syntehtic and got more than 200,000 miles out of all engines, while changing the oil only every 10,000 miles. In fact, the engines on all four were running strong when I got rid of the cars.
2. I filled my '77 Dodge 440 Motorhome (16000 lbs empty weight) with Mobil 1 and drove it for nearly 3 miles up STEEP hills in the Badlands National Park with NO water after dropping a lower radiator hose before reaching a pull off in 95 degree heat. I melted an exhaust manifold, and roasted the transmission fluid, but the oil looked like new. I drove it from there to Vancouver, down to San Francisco and back to the East Coast without a problem. I am convinced that the oil saved the engine. No question about it my mind. A Good Sam volunteer, who is a bus mechanic for the Salt Lake school system, couldn't believe it when he helped me check it in Wall, South Dakota.
I can confirm that my motorhome burned (and leaked) synthetics about three to four times faster than conventional oil. I evetually switched to Wal-Mart brand synthetic to save money.
I'm a believer in synthetics. I don't care if I burn a quart a tank. I'm still filling my new (to me) 300 SD with synthetic. Heck even at $2.50 every 500 miles the extra cost is only a $500 per 100,000 miles, or about $150 a year the way I drive.
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I believe the recommended change interval is 25,000 with a new filter and a top off every 5000.
I largely agree with Paul, vis-a-vis synthetics. They are superior in many ways. He didn't mention there very significant advantages in cold climates.
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No. 3 sums it up nicely. Much slower degradation (which gives rise to what was said).
DAS
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