The other day I was at a local bike shop complaining of the price of the Triumph recommended oil and the distance I have to drive to get it. The owner recommended Morris Lubricants Racing TT in 10w40 or 10w50. He claims that Triumph runs it in their factory race team bikes. I know the manual says to use 15w50 but was wondering if the 10w50 would suffice in my Speed Four. BTW, it's also half the price of the Mobil1 from the stealer.