I've never really understood this argument, the oil doesn't care about the ambient temperature, just the operating temperature of the engine. The manufactures recommendation is based on the engine operating within its normal operating range and the minimum temperatures seen during cold starting. If the cooling system is working correctly, the "hot" engine temperatures should be within the normal range. Starting a cold engine in 110 F weather is probably less challenging for the oil than -5 F weather, because it is closer to its operating temperature.

I agree, you don't need 5W40 under these conditions and 15W40 is probably more than adequate. I don't understand the logic of using xW50 in these engines.

My original (617) manual shows xW40 and xW50 oils being used interchangeably at higher temperatures. However, it indicates a minimum temperature of +32 F for the 20Wx oils; a minimum temperature of +23 F for the 15Wx oils; and a minimum temperature of -4 F for the 10Wx oils. Only 5Wx oils are show below -4 F. AFIAK, 5W40 was not available in the early 80s, so it's not listed. I assume the minimum temperature limits would be similar for your engine.

Based on the quality of current oil, I would use either 15W40 dino or 5W40 synthetic depending on cost and oil change interval. I guess you could use a 10,15, or 20W50 if you can find one rated for diesels.

Hi Craig,

The oil DOES care about the ambient temperature..... but only for starting.

I wonder what the highest Cx rating that any non synthetic 10W40 oil has. The highest ratings in the past have been in 15W40 oils but with the improved addatives, there may be much better 10W40 petro oils now than the best 15W40 oils of 20 years ago.

Personaly, I think if a modern engine needs an x50 oil, there is something wrong with it.

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY

I agree, that's what I was trying to say..... poorly. IMHO, cold weather starting is going to be more limiting than hot weather operation. I wouldn't put xW50 in any modern engine either.

Wow - thanks for the helpful info!

Let me make sure I understand so far...

20W50 is probably not necessary for my 350SD in Las Vegas because modern oils, especially synthetics, have good additives that will protect the engine at cold starts as well or better than heavier weight oils of the past... is that right?

Now, should I choose something like 15W40 since I'm not really concerned about any really cold starts since it only gets down to about 32F (at the lowest) in the dead of winter here in the desert? Will a 15W40 protect me enough at cold startups? Moreso than a heavier 20W50 oil?

I am convinced that synthetic is better for the car, so I am sure I will go that route. I am not interested in long-term oil changes - I want to change my oil every 3000-5000 miles, but I am not overly concerned about the expense of motor oil since I am running on WVO and I do not have to buy fuel very often - just when my main diesel tank runs low which should be about every 6-8 weeks. I am interested in saving money, but I do not mind the investment in more expensive/higher quality motor oil if it will help my motor run better/longer.

I am thinking of trying Schaeffer's oil since it is rated to be a very high quality and high temperature oil. At the risk of starting an oil brand name debate, I apologize that I am bringing up a brand name - but I would appreciate your thoughts on the idea! Thanks, I'm still learning - thanks to you folks!

I agree with what you've said. It's the lower value that's important for cold starts, so 15W40 is better in the cold than 20W50. Most folks here who use synthetics use diesel rated 5W40. I'm not familiar with Schaeffer's oil, is it rated for diesels?

From what I've read, Schaeffer's oil exceeds General Motors, Ford, Chrysler, and all foreign automobile manufacturer's specifications - it also exceeds Navistar, Cat, Cummins CES 20078, Detroit Diesel, Mack EO-N Premium Plus 03, and Volvo specifications.

Now that I look more specifically at weight, I have only been able to find Schaeffer's in 10W30 or 20W50. I'll have to see if it is available in 15W40... if not, would one of the two work - or should I keep looking for a synthetic 15W40 in another brand?

You mentioned most folks here use synthetic 5W40 - that is also what Brian Carlton recommended, and I have come to respect his opinion on things like this. I just want to get as much input from as many sources as possible because (as I have said many times before) I want keep this car running as good and as long as possible.

That sounds interesting, is it a full synthetic? What do you know about price and availability?

I wouldn't use 10W30 in your summer climate. It might be worth looking into the 20W50 for very hot climates if it's high quality oil. I still like Mobil 1 5W40 for year around use in pretty much any climate. I've used it from -5 F to over 100 F.

I think Schaeffer's is full synthetic. Here's some more info I found:

"Schaeffer's engine oils are synthetic blended from 100% pure parrafin base and the finest synthetic base stocks. This oil will withstand heat up to 250 degrees higher than conventional motor ols and will start easier than conventional 5W30 oils. Schaeffer's motor oils are specially blendedwith two proven friction modifiers, Penero and Soluble Moly, these increase engine life, and reduce engine wear by adhering to internal engine surfaces and remaining after shutdown which provides internal engine protection during initial start-ups and improves fuel economy."

I'll see if they have any other weights available, too.

Delvac 1 5w40 is just fine for any operating temps that these engines will see.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Go to BITOG and check the UOA of Delo 400 and Delvac Super, you will see that they do excellent vis a vis engine life etc. as good as the synthetics.

I have tried to use 20W-50 oil OM616 during summers here but found that the engine ran too heavy, it was as if the engine was running with pudding instead of engine oil.

__________________
99 Gurkha with OM616 IDI turbo

2015 Gurkha with OM616 DI turbo

2014 Rexton W with OM612 VGT