Full Synthetic 15w40 Oils

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May 31, 2006
Hi everybody, So I'm interested in switching from dyno to full synthetic in my diesel Rabbit. I've never used a synthetic oil before. My owner's manual says to use 15w40 (or if it gets really cold, a 10w40). I'm interested in switching to a full synthetic for several reasons (in no particular order): 1. Extending my OCI. 2. To see if I can lower my oil consumption. 3. To see, first hand, whether I can boost my mpg with synthetic oil. 4. I only put about 6000-7500 miles on my rabbit a year. So, I think I could get away with an annual oil change. I live in southeastern Iowa. It is my hope that with a synthetic, I could leave 15w40 in year round (as opposed to switching to 10w40 every winter). Now, here is my question. After reading the archives of BITOG and doing some other research, I've come to determine that there aren't too many full synthetic 15w40 oils. In fact, from what I can tell there are only four: a)Amsoil b)Redline c)Royal Purple d)Neo e)Shaeffers -- not a full synthetic, but considered by many to be better than the full synthetics, am I correct in this analysis? **Mobile 1 doesn't seem to have a 15w40. Are there any other full synthetic 15w40 oils that I'm not considering? What are your thoughts on the best way to go here? It seems like people don't seem to like Royal Purple too much here on BITOG. Why is that? So far, it appears to be the only one on the list above that I can buy in my town (available at Napa). But I could order the others off the Internet, I suppose. Also, is Shaeffer's really as good as (or better than) these full synthetics? The cost of each oil is probably the least important option to me (don't want to waste money though). I'm a mpg geek, so if any on that list stands a better chance than the others to raise my mpg (even if only a couple %), that would be very important to me. Thank you for your input and thoughts! Bryan Walton
Any of the listed oils will do you fine in your Rabbit. However, even though your manual states the use of 15w-40, it was published before widescale availability of synthetic 5w-40 heavy duty engine oils, which would be more than great for your little Rabbit. You see, a 5w-40 synthetic oil will provide better cold weather flow properties, and will also protect better than conventional 15w-40s at high temperatures. It's the best of both worlds! Since cost is an important factor to you in your decision, let me be the first to recommend Shell Rotella Synthetic 5w40, which one can find easily at about any Wal-Mart in your area. (Purplish Blue Jug) It can be bought for about 14 bucks a gallon, and will provide long drains, increased mileage over dino oil, and great protection in hot AND cold weather. I don't feel that in your application you are going to see much of a difference between Rotella Syn and any of the oils listed above that would offset the difference in price and/or availability. Now if you were to go to a bypass filter system and all the hoopla I would probably suggest a "true" group IV or Group V synthetic like those mentioned above. For the type of drain interval you desire, the Rotella Syn will be more than adequate, and cost effective. When this Rabbit was built, it would have loved to ran on an oil like Rotella T Synth it's whole life! I would also say Schaeffer's 7000 blend 15w-40 (which would be okay winter and summer) if you had a dealer close where you could buy just the quantity needed and not the minimum $250 order needed for delivery. Mobil 1 Turbo Diesel oil is hitting the shelves at wally world as we speak and would also be a great alternative, albeit a little more pricy.
I agree that a 5w40 makes better sense, for better start up conditions. I currently run a 50/50 mix of Mobil 1 T&S 5w40 and Delvac 15w40, but will probably go to 100% 5w40 this next winter.
Thanks for the replies guys! I must say, you have opened the flood gates for me (regarding my options). I had never even considered swaying from what my owner's manual and Bentley Service manual had always told me. And that definitely gives me more options for choice in motor oil. Thanks! I have several sources in town for the Synthetic Rotella 5W-40, and also Mobil 1 5W-40. In fact, I also have access to Mobil 1 0W-40. What are your thoughts on the synthetic Rotella vs. the Mobil 1? And the Mobil 1 5W-40 vs. the 0W-40? It seems like the Mobil 1 0W-40 might the holy grail of diesel motor oils, would this be a correct assumption? BTW, all of the Mobil 1's are on sale right now for $4.49/qt. Thanks! Bryan Walton
Oh, one more question. My owner's manual has a little graphic that shows the climate operating ranges for a few of the various oils. For the 20W-40 and 15W-40 oils, it shows the upper range for outside ambient temperatures to be around 85-90 degrees fahrenheit. However, for 10W-40 it shows the upper range for outside ambient temps to be around 50 degrees. Is that a mistake or does 10W-40 really not work in warm weather as well as 15W-40? The reason that graphic concerns me is because it makes me wonder whether 5W-40 and/or 0W-40 is similar in that regard. Thanks! Bryan
Bryan, While the 0w-40 would work fine in your application, and would be much, much, much better than what it has ran on most of its life to this point, I would stop short of dubbing it "the Holy Grail of Diesel Oils." And here is why: While it does carry enough diesel spec for a motor the age yours is, it is really an oil designed for higher output gas motors first and just happens to be tough enough to meet older diesel specs. The 5w-40 diesel specific oils will better handle soot and stress on the oil than the 0w-40 will, particularly in long drain scenarios. To answer your specific question on Mobil 1 vs. Rotella T synthetic--I would give the nod to M1 5w-40 over Rotella T Syn, because it is a "true" Group IV/Group IV based synthetic oil, whereas Rotella T Syn is a Group III "hydrocracked" petroleum based oil that can legally be called synthetic in the U.S. but truly isn't "synthetic". Does it mean it is bad oil? No, this is a great oil, and is a good value. It just irritates some of the "purists" here on BITOG who prefer their synthetic oils be of Group IV/Group IV classification. Both will flow well when cold, both will perform well when hot, and do their jobs accordingly. Both are WAY more than adequate for protecting your engine. M1 may be just a tick (and I mean a tick) better at the extremes, and may extend drain intervals a smidge longer if you were going say 15-20K miles per drain. But in your application the difference between the two will be so small as to be considered negigible. If you want to spend the extra 6-8 bucks on M1 and it makes you feel better at night, go for it. If you want to keep that extra 8 bucks in your pocket by choosing Rotella Syn and use it buy 2.5-3 gallons of diesel that will take you 80-120 miles, then by all means do so. We normally run M1 5w-40 in our TDI Jetta, and have run Rotella T Syn as well--I could tell no difference at all. I am switching to Schaeffer's 5w-40 on the next drain here in a couple weeks and will compare Used Oil Analyses. About your 10w40 concerns with temps...Basically, most 10w-40 oils are not diesel rated, or diesel specific, and due to the wide viscosity spread, dino based oils require a lot of viscosity improving polymers to make it perform like a 10w when cold, and a 40wt at operating temp. At high temps and stress, these polymers tend to break down,("shear") degrading the oil and ultimately shortening your safe drain interval, and generally thinning the oil out of grade, which would make it less than ideal for higher temps, etc. Read about oil shear here: http://www.bobistheoilguy.com/oilshear.htm Because synthetic oils by nature have better viscosity index ratings than regular dino oil, that means they need much less, or sometimes none of these polymers. This means they stay in grade much longer, have less issues with hot weather protection, etc. Plus since synthetics don't coke and burn at as low a temperature as mineral oils do, they don't leave the kind of deposits on the ringpacks that burnt minerals do. They also maintain their film strength at higher stresses and higher temps. So that 5w-40, despite the fact that it has a 5w instead of a 15w as the first number, will outperform the 15w-40 dino oil in hotter temps, and your Rabbit will definitely like the 5w portion better on a frosty Iowa morning in mid January.
Only 6-7K miles per year = an easy OCI = Rotella-T 5W-40, IMHO. Why get all O/C about it, anyway? Could probably run one of the older SJ/CD oils too. How many miles on your bunny? Cheers!
I just want to add (if not mentioned allready-I just skimmed thru the responses), that you stated "To see if I can lower my oil consumption.". Expect to see an increase in oil consumption when switching to synthetic. There are many reasons why an older engine consumes oil, some of them may be corrected by using an engine cleaner like AutoRX, but if the consumption is from just normal engine wear (greater than original tolerances), then once again, expect to see an increase in oil consumption. Still, I like the use of synthetic oils, and I tend to agree with the poster regarding Shell Rotella T synthetic. Biggest bang for the buck whith all things considered. Good luck, KD
Amsoil AMO 10-40 will fit your bill too. It's gas and diesel rated. I personally would go with the Amsoil ACD 10-30/30 straight grade...just say NO to VI improvers.
Hi everybody, thanks for the replies. I've ordered my first bottle of Auto-RX. I plan to do two applications of ARX and may then switch to synthetic. Cheers, Bryan
I have had a lot of experience with Rabbit diesels. We must be talking about a 76-84 car, right? Not the much more modern TDI engine cars. At our shop we always loaded the Rabbit diesels with 10-40W Redline. Not that the other oils are bad, we just happened to sell Redline. The engines loved Redline, the cars never died on it, unless the front crank pulley snapped of the crank! Not an oil related problem...Using the Redline fuel catalyst is worth the money. I would imagine that your best dollar spent on your Rabbit would be to get a bypass filter installed. Ralph Woods sells them for only $130 and you have lots of room for a simple installation in your car. No bull added, the research clearly show that you will at least double the life of your motor with a bypass filter installed.
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