Castrol RS 10-40 motorcycle oil compared to Mobil1???

Have a friend that would like to use the Castrol motorcycle oil. It's easy to obtain at Kragen and meets a higher standard than the Mobil 1 according to the specs on the bottles. Anyone know if it's a true synthetic and if is as good as Mobil1?

Castrol claims the automotive oil has 8 times the protection as Mobil 1.

I still use Redline in all my motors and unable to convince the person wanting to use Castrol.

Grey any knowledge????

"8 times the protection" of Mobil 1 engenders two reactions from me. The first is simply that it's a pretty outrageous statement, and the second is, "protection against what?"

Amsoil's most recent MC oil comparison test included both Mobil 1 MC oils, as it has for several years, and interestingly this time it included Castrol RS R4 4T in 5W-40. The Amsoil MCF and the Mobil 1 Racing 4T have always run near the top of the 40 weight group. The Castrol finished 7th of 15 oils tested.

The paper is here:

Note in particular the ASTM D6278 viscosity retention test on page 9. The Castrol actually did pretty well in this regard, and it's comforting to see that a good many more oils are passing this test than was the case just a year or two prior. The ability to stay in grade when used as a transmission lube is one of my biggest issues with oils not formulated for the purpose. The Castrol also did well in the gear performance test (D5182), which is kind of a pass/fail thing.

But looking at things overall, it stunk in the High Temp/High Shear test, the 4 Ball Wear test, and wasn't the least bit stellar in corrosion protection, acid neutralization, or oxidization resistance.

Based on that, I wouldn't use it myself, but people use worse oils all the time and get away with it. As to the "true synthetic" question, that's a valid point to raise given Castrol's rather fast and loose history with that term, and the whole issue of Group III oils. But the paper refers to the oils only as synthetic or petro/synthetic blends without specifying the base group beyond that. I do know that both Amsoil MCF and M1 Racing 4T are Group IV bases.

It's worth mentioning that the original "8 times" claim did not specify which Mobil 1 they were talking about, and not all of the M1 oils will do as well in D6278 as the MC blends do. That may mean something here, or it may not apply.


Love to see Redline part of that test. Pretty sure I know why they are not.

Redline could always step up and do their own, but they haven't. What I would really like to see is some of the vaunted low cost "solutions" like Rotella and Mobil 1 EP tested against this group.

So would I. I cringe when I hear people using those oils. They may work great in diesel motors but Most diesels also have a min. of 15 qts and an oil cooler. What most people are missing is that most dirtbikes hold roughly 1 qt and the oil gets abused from high heat and high rpm's. This is where a true synthetic shines. Like I have stated before, that's why I buy new bikes and not used.

Heres an old oil report on Rotella T Synthetic in a street bike.


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