It's reasonable that the cold temperature viscosity can tolerate a decrease from 15W to 5W with increasing synthetic since the synthetic is a much better lubricant. Less wear and tear on cold startup.
But why does the operating temperature viscosity for the blend drop to 30 and go back up to 40 for the fully synthetic?
I've noticed this for other brands of oils for diesels. Don't kick me out of here, Cletis! I've asked this on a diesel forum and the post gets hijacked by an oil salesman.
Last edited by Wally / Montana; 02/24/20155:43 PM.
34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
I used 15W40 in my Cummins 14 Liter as well as my 10.7 3engines for over 35 years. I won't mention the brand. I sell Rotella as well as other brands. I had been changing oil at 15,000. I recently switched to synthetic blend and my oil samples still look good at 20,000, And the gauge shows pressure noticeably quicker on cold starts. I cm contemplating switching to full synthetic in my trucks. Possibly buying a 55 gallon drum instead of of 300 gallons in bulk. I use 5w30 full synthetic in my GMC 6 liter.
European engines are very strict about speccing 0W40 and thats what I use in my European engines. Europeans put a lot more effort into engineering instead of meeting emissions like we do.
I will not take a step back to petroleum based lubricants and will probably step up to full synthetic. I have been using synthetic in my transmissions and rears for at least 25 years. We used to rebuild them every 200,000 miles now they go over a million miles if nothing stupid happens to them.
A video that helps explain what and how a multi-viscosity rating is and how it is measured. The interesting part for me is the "cold" test. The difference between synthetic and other oils is not discussed on this one.
For some information on diesel rated oils this may be helpful. http://www.machinerylubrication.com/Read/28576/comparing-gasoline-diesel-engine-oils- There is a lot going on in your oil that we don't see. Concerns about catalytic converters is only a part of the issue. The temperature that is used in determining viscosity can make a huge difference. Diesel oil is rated different than gas engine oil. I for one don't know a lot about synthetic oil but the makeup in chemical composition will have some effect on mileage/fuel consumption and the contamination levels are I think the biggest differences. I suspect the idea that synthetic is more "slippery" comes from the better fuel consumption with synthetic oils. The fact of life for diesel engines making different contamination in the oil is easy to get. The idea that the oil might change emissions is less obvious. So have I helped? Or did I just open a giant can of worms?
I was mostly wondering about the higher temperature 40 (conven) dropping to 30 (blend) but back up to 40 (full synth). Perhaps if you shoot for a 10W cold viscosity by blending conventional (15W) and full synthetic (5W), the higher temperature viscosity just happens to be 30.
OK, I know I'm being a bit @n@l here but I like things to make sense.
34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
I am not a Mobil distributor. But I think their products guide is very self explanatory as far as synthetic oil goes. Many people don't realize just how many synthetic oils there are for specific purposes.
The "weight" of any oil is determined by time through a metered orifice at a specific temperature. Conventional, Blended, and full synthetic. The only weight involved is the weight of the volume above the orifice. The cold test determines the lower number on multi-viscosity oils. A 20-50 oil flows like a 50 weight oil but has a cold test result of a conventional 20 weight oil. I have no idea what blending different "weight" oils might produce. I would also have no clue what would happen if you blend a Conventional oil with a Synthetic. Some labels want a complete change out. Drain oil, replace filter before you add their Synthetic to the engine.
Steve_H is correct. The 'weight' of an oil is derived at the time it takes for a set amount of oil, at a SPECIFIC temp to pass through a measured orifice.
The time is then cross referenced on a chart. The chart has SOME overlap.. and the times are not really that far apart.
You will see that for SAE 0W, 5W, 10W, 15W and 20 (non-W) the difference is all within 100 points.. out of a scale of a possible 10000 (or 1500 for common grades).
Sorry about my poorly worded question. It had nothing to do with the definitions of viscosity or how it's measured. I measured viscosity eons ago in a physical chemistry lab.
I just found it curious that the higher temperature number went from 40 to 30 to 40 with increasing proportion of synthetic. Go figure.
34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
Wally / Montana Like you I am far from an oil expert. I suspect the reason they can make a 0w40 full synthetic but only 15w40 or 10w30 dino oil is related to the level of flex afforded by the synthetic process allowing a wider range between hot and cold viscosities. The narrower range possible in dino oil prevents keeping the hot 40 as the cold number is lowered.
