TimVipond said:
A few reasons I can think of:
1. My 2006 manual states API classification SF, SG or SH. Most car/truck oils are SJ, SL, SM. These later classifications are formulated with different formulations
2. There is a BMW service bulletin that recommends against SJ oils..
However, IF an oil lists the above grades, even if it meets SJ requirements it also meets any others listed. For instance: Castrol Syntec synthetic lists: "Exceeds all car and light truck manufacturer's warranty requirements for the protection of gasoline, diesel and turbocharged engines where API SM, SL, SJ, SH, CF or CD is recommended"
3. I prefer to use an oil and filter that is warranted for parts and labor should the oil or filter fail at up to double the OEM change interval or 1 year, whichever occurs first, which I used in my 137,000 mile 2000LT. I don't know of any car/truck oils that have a similar warranty for motorycles. The manufacturers themselves do not test/formulate/recommend their oils for use in motorcycles, and may not cover you should problems occur. Call them up and ask them.
4. I've noticed many car/truck oils tend to cloud up the oil level sight glass.
5. The sprague clutch which is splashed with engine oil is touchy as to what comes into contact with it.
The Sprag clutch needs lubrication, but it is not "touchy" in that respect. However, it acts as a small centrifuge, so any cabon buildup on the sprag cage causes the sprags to stick and not engage the hub properly. It is not a lubrication issue, but a sticking one. My sprag clutch stuck after thousands of miles running with broken piston ring lands allowed a lot of blow by, causing carbon buildup in in. There was no wear to speak of evident in it, and I could have just cleaned it and re-installed it, but I had the new part so put it in.
6. Ask BMW if using a car/truck oil is a good idea. I did and they said motorcycle specific oils only.
Yet again you continue to bring this up when the person who answered your question totally destroyed any credibility he had with the comment about "damaging the clutch", which is certainly an issue with the NEW K bikes with the engine oil lubricated clutch, but
NOT on the LT which has a dry clutch.
A good source of information that gives reasons to use motorcycle oil over car oil and compares 26 motorcycle oils in 10 standard ASTM test methods can be found at
https://www.amsoil.com/lit/g2156.pdf .
I do not argue at all about whether or not Amsoil is a good oil, it is. However, I still very strongly believe that motorcycle specific oil accomplishes absolutely nothing in the "brick" K engine. Any GOOD automotive oil that meets the SG/SH requirements in 20W50 grade will fill the requirement nicely. There are a LOT of very high mileage BMWs on the road that do not use "motorcycle" oils. At 100,000 miles when I had to open up my engine due to the detonation damage to the pistons, there was practically NO measurable wear on ANY engine part. I put the original crank and rod bearings back in as there was no reason to change them. Even piston ring gaps were barely at the end of the INSTALL tolerances, had not even encroached on the allowable wear range yet!
I may have tried Amsoil myself, but saw no need to since very good oils are readily available at my local auto parts stores that will easily meet ALL requirements for the LT engine. Amsoil is not usually readily available in your neighborhood, which to me is it's greatest drawback. I can get very high end European synthetics easier here in San Diego, such as Motul and Lubro-Moly (Liqui-Moly). I like being able to walk in and pick up oil "off the shelf". However since the LT engine is basically an automotive design in all respects, felt no need to use other than off the shelf high grade automotive oils.
There would have to be evidence on many very high mileage LTs that used various oils to show any real advantage in using expensive oils. The real fact is that very many, if not most high mileage BMWs have used standard automotive "dino" oils, not even synthetic, so the "advantages" of any brand/type over another in the long run are very small at best.