I use either Rotella T, HD360 or Syn3 in the engine and Formula+ in the trans and primary, it's what HD recommends and I can buy it all at the same place along with a filter.

I used Castrol GTX 20-50 in my BMW's for years with no problems so I might start using that as it's cheaper.

 

Will since starting this I've talked to several HD shops, got several answers but in the most part syn3 in the motors but a lot say not in the 6 speed trans, they say it would be okay but not as good as a good primary fluid! so still up in the air to which way to go, looks like their are a lot of opionions on this subject!

 

Wally if your undecided about the tranny lube, you just can't go wrong with a quality semi-syn or synthetic GL-5 spec 75-140 gear oil in the HD 6 speed.

Personally I wouldn't run motor oil (syn3) in the tranny, but that's just me and to each their own.

Some will say 75-90 and that's probably fine in temperate weather conditions but for hot hot hot hard riding it's 75-140 for me.

 

I have used only Syn3 on my present bike and the 03 Softail I used to have. Oil works fine and have had no engine issues because of it.

 

Just my own observations, so do what you are inclined to do. But you asked so here's what I found in over 120,000 miles on various Harleys since 2004. Every one of them had the 1000 miles done by the dealer at or before the 1000 mile mark. The earlier ones were HD dino and the last 3 were the recommended synthetic in all three holes. By 3000 miles I put either Mobil 1 80w 90 gear oil or most recently Spectro 6 speed oil (which is 75-140) and either oil makes shifting smoother, quieter, far less fifth gear whine in the six speeds. Easier to find neutral and the operation doesn't change but very little in 10,000 miles I run between trans oil changes, nor a significant change in 100+ degree outside air temps. With the HD SYN 3 the bike sounded ugly in hot temps and stayed that way until you changed it. Every shift gave a metallic tink. Shifting started getting louder and more metallic by 3000 miles with HD crap, dino or SYN 3. If it was very hot outside air temps even quicker.

I bought the wife a FLHXXXtrike last August 09 and she had a helluva time ever finding neutral when new and after the 1000 mile maint with syn 3. At 3500 I did the maint myself and added Spectro 6 speed oil to the trans, Mobile 1 V-twin 20-50 in the other two holes (6.50 a QT from the Distributor) and she has NO PROBLEM at all finding neutral and even though this was her first bike/trike ever to operate she asked what I did that made it so much quieter and easier to shift. Oh and I only change the mobile every 5000 and the Trans every 10,000. By the way she has 12600 miles on it now. I couldn't wait to get the SYN 3 out of any bike I had it in by 3000 miles as it got so darn noisy and changed so much with heat I hated the stuff. Will the SYN 3 make your bike wear out faster? I doubt it, but I just don't like the feel of operation nearly as much as the Mobil and Spectro combination. It makes me feel good!

 

I run schaffers oil in the motor. 20/50 racing synthetic. About 5 dollars less per quart and It holds oil pressure, after a good long ride on a hot day, 100 times better then Syn3. My motor doesn't sound like it is going to come apart now either. I run Redline in the trans and it is smooth as butter and quiet as a mouse. Mobil 1 synthetic atf in the primary and it now runs cooler then before and it also made it much smoother shifting with better clutch performance.

Ran Syn3 for the first two changes. Hated it. I could **** a better product and it would cost much less.

 

Never noticed this before, but Syn 3 is certified to meet "HD requirements" but there are no API certs on the bottles.

 

It's probably not necessary as it is marketed for use in HD products so the "HD requirements" statement is adequate.

 

Currently running Valvoline V-Twin 20-50 in the engine and primary, Bel-Ray Gear Saver in the tranny. I used to run Amsoil in the engine but stopped using it due to noise issues. A lot quieter with the Valvoline. So far, so good. I bought the Valvoline at AutoZone on sale for $3.99 a qt. Just had to try it. The Bel-Ray really helped with the shifting and noise. I am going to try the Valvoline Synthetic Gear Oil and their ATF in the primary at the next service. Valvoline has been around for many years and I can not find a reason why their product would cause any failures. Been using it in my autos for years.

 

As it states in the owners manual. If it is necessary to add oil and HD oil is not available use an oil that is certified for diesel engines. Acceptable diesel engine oil designations include: CF-4, CG-4, CH-4 and CI-4.