Being one of the people that owned several Kenworth trucks, let me assure you they do not use 140 weight oil anywhere! The absolute best combination I ever found was plain old Rotela 15/40 CONVENTIONAL oil in the crankcase, even in my 600 horsepower Cats, straight 50 weight motor oil in the transmission, except for the 18 speeds and they got Eaton 50 weight synthetic oil and 80/90 synthetic in the rear axles! Those combinations ran 50 degrees cooler oil tempratures than anything else I tried, and let me assure you there were several oils tried in my fleet over a 30 year career! My reasoning for commenting is if CONVENTIONAL oils can stand up to a 600 horsepower 2800 foot pound torque powertrain in a truck running at gross weight day after day, and often for 1 million miles WITHOUT FAILURE WHY do folks think they have to run some exotic oils in their 60-80 horsepower 100 foot pound Harley drivetrains? My own Harley gets 20/50 conventional oil in the engine, 15/40 in the primary and 80/90 synthetic GEAR OIL in the transmission and it gets changed every 3,000 miles. My last Ultra had 108,000 on the odometer when I traded it without a powertrain failure.

 

I know it's an older thread but I should add to the database that I've been using Rotella T6 in my primary since I had my Road King in 2011 and everytime I crack open the primary case to inspect my Attitude Adjuster for excessive wear, its looks brand new in there. Granted I never go more than 3-4k on changing it out. I put Formula+ in my Roadster when I did my 2k service but I've got a quart of the T6 ready to take its place in the near future. The primary and the tranny share oil (dont care for that engineering but it is what it is) so having a solid lubricant to share between the two components is vital in my opinion .

 

I have used Formula Plus for years in the Transmission & Primary, I put Royal Purple in my Engine
In my opinion, Rotella is a good oil to use in Primary and Transmission most cases, anything used in diesel industries I am good with.