Per my previous post Formula+ falls within the viscosity range of a straight 50W.
And yes, the transmission can use 20W-50 or 75W-90. In fact, most manual transmissions in automobiles these days are running 10W-30 engine oil or, in some cases, ATF.

 

So if I interpret all this correctly, and my preferred fluids are Mobil 1, I run synthetic 20w50 or 15w50 in the crankcase and synthetic 75w90 in the drive and tranny... yes?

 

Yes, for the engine & tranny. For the primary, use engine oil or Formula+.

 

I run redline shockproof heavy in my trans which is a 75w250. Takes 20 minutes to pour it in.

 

Formula+ is an unexceptional fossil engine oil with a small amount of EP for the tranny, according to someone here who had it tested. For that you are certainly getting ripped big time for the $6.50 you're investing, especially since the primary needs little more than a cheap fossil engine oil to do its job. I don't skimp on engine or tranny oil and use a high-quality synthetic for both, but the primary gets 10w40 I get at Wal-Mart for $1.24/qt. I used this and other brands of fossil engine oil in my '96 RK and had no trouble for the nine years I owned it.

I suspect that Formula+ is of a lower viscosity than 20w50, as when I consulted with an HD engineer a year ago he was hesitant to okay the use of 20w50 Castrol GTX in my primary to quell the clattering experienced by most '07 owners (later fixed by IDS). He said 20w50 was too viscous for most climates, implying the Formula+ was lower. I used the GTX for a while, then moved to the lower viscosity oil, and I didn't notice any difference. The GTX did seem to quieten the clatter slightly, but as I said IDS fixed the problem for good and there was no need for 20w50 from that point on.

If you must use a synthetic, find one that has no moly, as being a friction modifier it may cause problems for the clutch pack. I've never seen any evidence of this, but the concept makes some sense. Some use Redline MTL, which I think is severe overkill, but it has no moly in its formula and is recommended by Redline for the HD primary.

 

For all you Formula+ fans, Latus HD is running a special on it you might like.

 

I don't understand, $9,194 for 330 gallons calculates to $27.86 per gallon or $6.97 per quart. Doesn't seem like much of a bargin to me.