My new FLHTK 2015 MY maintenance guide states I can use full synthetic 20W50 in the trans. Can I use the same in the primary?

 

You can but why waste the money; run any good 20W50 or even a 15W40 conventional. Also run a gear lube in the tranny, not a motor oil; something like Redline Heavy Shockproof or any quality 75W90 gear lube like Redline, Amsoil, Mobil 1, Spectro, Bell Ray, etc. Synthetic motor oil in the motor, gear lube in the tranny and conventional in the primary, or Amsoil and Redline both make lubricants specifically for the primary.

 

I went with Mobil 1 104361 75W-90 Synthetic Gear Lube for the trans and
Red Line 42904 V-Twin Primary Case Oil for the chain case.
How's that combination?

 

If you run synthetic 20w50 in the primary, make sure its motorcycle specific since regular synthetic can cause your clutch to slip.

 

SNY will not cause any issue in primary. This has been beat around for years and to this day no one can show that it does. many have logged a lot of miles running syn in primary . Not really necessary but nothing wrong with it.

 

Very good choices. The thing I like about Redline Shockproof for the tranny is the pink color which makes it easy to identify whether a leaking lubricant is from the trans or the primary and the leaks show up between the primary and the trans case and it is difficult to determine the source if both fluids are the same color. Touch wood that neither of us should have a leak.

 

I use Mobil 1 20/50 V-Twin in all 3 holes. Engine, Primary and Transmission. 30,000 miles no issues.

 

Valvoline VR1 Racing Synthetic will make your clutch slip. I know this from personal experience.

The best stuff for your Primary is Formula +.

 

Ai also run mobile 1 v twin ..I run this in all three holes 2008 fatboy 30000 miles no problems at all ..and found at autozone with K@n oil filter also at autozone

 

No disagreement that F+ is a fine product and is all you really need it the primary. Been in my 1996 all of it's life. Just the same nothing wrong with SYN 20/50 either.
To keep life simple on the RGU I use M1 20/50 in engine and primary. Also use it in the 750 Street one fill takes care of everything on that one.
I have never had a clutch slip with SYn in primary.