Here is a letter from Castrol regarding the use of Diesel oil in any engine other than a Diesel engine. This letter was obtained by another member so I am just reprinting it here for information:
Although Castrol Diesel oils carry an S rating, its primary credentials are diesel. The S rating is meant for short term use if the owner cannot find the appropriate gasoline engine oil. Diesel engines and gasoline engines have different operating regimes (loads, temperatures, speeds, fuel, exhaust recirculation); the lubricants are formulated differently (additives, ZDDP) to be in line with the primary operating requirements of the engine. Diesel engine oil is formulated with very high levels of detergency to counteract the soot that is circulated back into the oil. Detergency is surface active and does compete with ZDDP for the surface of the cam lobe and lifter face. The proper engine oil to use long-term in gasoline classic car engines is an oil designed for use in a gasoline engine with the correct level of ZDDP to protect his cam and lifters.
So, apparently, you do not need the very high levels of detergent in a Diesel oil for your motorcycle, and the detergent impedes the action of ZDDP, which you do need.
I have tried a variety of oils over the years but since the mandated reduction of ZDDP for pollution requirements, it is now even more important to use a motorcycle specific oil. I am currently using Valvoline 4 Stroke 10W-40 in my Honda four cylinder bikes because they have Babbitt bearings on the crank and a high pressure (65 PSI) oil system. I use 20W-50 in the Yamaha XS650 because it is a low pressure oil system (5 to 15 PSI) with ball bearings on the crank.
The Valvoline 4 Stroke is JASO MA API SF/SG/SJ certified and is available at my local Advance Auto dealer, sometimes on sale including the filter I need for the Hondas. Non sale price is about $5.25 a quart. I change the oil more frequently on the XS650 because the ball and roller bearings actually crush the molecular polymers that give the oil it's "multi viscosity rating, leaving the oil at or near its base viscosity of 20 well before the oil has gotten dirty.
The Hondas can go a little longer because they both have built in replaceable full flow oil filters and the Babbitt bearings do not crush the polymers.