Didn't want to hijack Jose's oil thread.......

[Zeuk in Oz]

But can anyone enlighten me as to the different requirements and hence different recommendations of oils for diesel engines as opposed to petrol (gas) engines ?
I run my '88 525i M20 (235kkm / 150kmi) on Mobil 1 5-50W. I have never seen 0-40W in Oz. I have owned the car for 2 years (30kkm / 19 kmi). It does not leak oil and does not require topping up between service intervals of approx 10 kkm (6kmi). I have no idea what oil was used in the car before I bought it.
My Diesel ML 270 CDI (100 kkm / 62 kmi) is still being serviced by the dealer and they have used Shell Helix Ultra 5-40W since new. (Full synthetic). Service assyst gives service intervals of approx 24 kkm (15 kmi). What surprised me though was that they were using a "petrol" engine oil in a diesel. I had always thought that there were petrol engine oils and diesel engine oils and that they did not cross over.
Can someone explain this to me ?
It is interesting to note that my local BMW dealer also uses Shell Helix Ultra 5-40W in BMW diesels in the X5.
This brings me to my third car and my problem : I have a 7.3 litre V8 Diesel F250 4WD crew cab with 33kkm (20 kmi) on the clock. The dealer uses a semi-synthetic Elf oil.
Can I use a full synthetic in this engine ? I have never seen Mobil 1 for SUVs in Oz. How does it differ ?
Is there a fundamental difference in requirement for diesels as opposed to petrol engined cars or is it a matter of "oils is oils" ???
Please explain !

[ryan roopnarine]

the very, very short answer to this is....if the back or front of the oil bottle says meets xxx oil standards for (mb/bmw/ford/catepillar) diesel, it is an acceptable diesel oil. diesel oil usually contains more dirt control components since diesels naturally have more particulate as a product of combustion than gas engines do. diesel oils might have a different additive package in certain hemispheres/climates because of differing catalytic converter construction/lack of catalytic converters....these cat. ingredients might be sligtly harder on a gas engine's cat than a gas oil is supposed to. i'd go to the place you normally purchase engine oil from and look in the section that has traditional diesel oils (they are usually seperated somewhat), and work from there. for example, in N America, the two main diesel oils are Chevron delo 15w40 and shell rotella 15w40. the full syn would be below these oil, marked Full synthetic rotella or chevron delvac 1 (full syn). diesel oil or such. that's how i'd determine such. in your f250, IMHO (and its not worth much) i wouldn't put anything other than a branded diesel oil into there, not passenger car stuff. up to you whether or not you want that to be a synthetic. oh, and FYI, most places on earth do not use as heavy an oil as is marketed in Oz and such, 50 weight is considered obsolete here, that's why mobil 1 is sold as 0w40 as the fattest, and 15w50 here. please understand this if you feel that an oil you use on a regular basis isn't getting enough talk time on the board. if you are looking for a dedicated diesel oil, check the back for a glut of diesel certifications and a lack of gas ones, or look below the diesel oils in the store.

[BigKriss]

I have never seen 0-40W in Oz.

I have at K-Mart.

[Gene in NC]

Want to have a fist fight with a semi driver? Try to convince him that Shell RotellT is not good oil. THat's what I wound up with inn GM 6.2. Dunno bout best syn.

[Zeuk in Oz]

Have spent some time talking with retailers about this issue and it is quite clear that some of them are as ignorant as I am.
I would imagine that Ryan's response is correct - to use what the manufacturer recommends - but still keep thinking about it.
What I am being told, however, is that the reason for semi-synthetic oils is twofold : firstly to stretch the viscosity range and secondly to increase the time between changes.
What I have difficulty understanding is why a mixture of mineral oil and synthetic oil doesnt respond as 2 different oils in the one space. In other words surely the minearal oil degrades as quickly as it does without the synthetic there with it. Therefore you would be left with a situation that after that happens, only the synthetic oil is giving protection.
It would also appear that the characteritics of synthetic oil cannot be altered as can those of mineral oil. In other words you cannot make a synthetic oil have a more detergent character than it already has. Does this make sense ?
Also full synthetic oils such as Mobil 1 and Shell Helix Ultra are now considered OK for small capacity diesels but not for larger capacity diesels. Would this be because tolerances in the larger engines are less exact?
Or is there some other fundamental factors that I am missing ?
I sought advice from Mobil 1's US website for my F 250 and they recommend Mobil 1 Truck & SUV 5W-40 oil, but as far as I can determine it does not exist in OZ.
On the OZ Mobil 1 website they recommend Mobil Delvac 1 or Mobil Delvac MX 15W40. Apparently Mobil Delvac 1 is fully synthetic. This oil is also recommended for my ML 270 CDI - no mention of straight Mobil 1.
(Big Kriss, you were right by the way, Mobil 1 0W-40 is available in Oz but I have yet to see it. )
This leads to the next question - given that they are all full synthetics, how does Mobil 1 5W-50 or 0W-40 differ from the Mobil 1 Truck and SUV 5W-40 or Mobil Delvac 1 5W-40 oil, apart from the obvious differences in viscosity ranges ?
Sorry to be so anal, just trying to cut through the b***s***.