No, what the dealer told you was true and correct. The MoCo says that you can use their Syn3 in all 3 holes. That said, you can use any synthetic 20/50 oil in the same application. If this were not true, then the MoCo would have to supply you with Syn3 free of charge.

Many owners use other 20/50 synthetic oils in all 3 holes without issues. Others choose to use application specific oils in the primary, tranny and crank.

 

Ok, thanks. So far the vote is 1 for and 1 against. After some searching I will see what I come up with.

Any disadvantage to running heavier gear oil in the tranny?

 

There was a post that I read yesterday on here about engine oil and several tests that were done. Here is the link that explains what some of this stuff might mean. It goes into specific oils but also tells you what you might look for in your specific oil. The thicker oil should not cause you any problems. You really dont have to worry about the oil being pumped in the transmission. The tranny is a wet sump and the oil just gets flung around. Even when cold the only problem that you might have is an every so slightly harder start but it wont be noticable.
http://www.amsoil.com/lit/g2156.pdf

 

When I first went to synthetics at 2000 mi. I put M1 85/140 in the trans ($16 qt.). When I changed again at 5000 mi. I went with M1 75/90 to try it and save a little money (about $10 qt.). I like the 85/140 better and will change back at 10,000. It just seemed to shift smoother with the heavier oil.

 

i just replaced my factory fill with Redline V-Twin Transmission Fluid sunday in my 09.
I had used it before in my 01 so i knew what to expect. It definitely works. It is the equivalent of 75W250 when warmed up.