Gear oil and motor oil are rated different. Believe it or not those 2 are pretty close in weigh

 

One other thing you might consider is after the oil has drained out, pull your spark plugs and spin the motor a couple of revolutions with the starter. That will pump the 6 or so ounces of oil left in the motor back up into the oil tank where it can drain out. This isn't critical, but if you're going to use synthetic (which I strongly encourage), might as well get all the old dino stuff out that you can.

 

The friction modifier debate seems to go both ways. Penzoil vr-1 has moly in it and it seems to be the choice of many jap bike owners and from what I have read no one is reporting a slipping clutch. Personally I wouldn't take the risk. You can use any oil really for the motor as long as the last number is 50. The primary is where you shouldn't in my opinion use an oil with a friction modifier in it.

 

Also. A 5w-50 will shear down quicker than a 20w-50 because of the use of viscosity improvers. I use a 5w-50 or 10w-50 in spring and fall. It gets changed out for a straight 50 for summer then back to it again. I use good oil and could stretch out the intervals but I am still trying different products to see which the bike likes better. So far red-line shockproof heavy is a winner for tranny,their primary oil seemed to quiet down a bit. Amsoil has been good to me but red-line also makes a great engine oil. I would rather an oil shear down than get thicker and almost everyone I know who has used mobil 1 v-twin 20w-50 has stated the oil was much thicker at change time. That will accelerate wear at start up. It is getting thicker that also means is suffering from evaporation and concentrating the contaminants in the oil.
In the end a straight cheap Dino 20w-50 changed at 2500-3000 miles will keep the bike happy and provide many years of service. I like synth simply because I can push it with no real fear of damage.

 

What is evaporating out of the oil, do you think?

 

Thanks for that info! I'll have to reconsider my stance on the big twins transmission oil now. That said, I still gotta do what the customer wants, even if it's not my opinion.

 

I went with Amsoil 20W-50 in the engine and stuck with the Harley Formula+ stuff in the Primary/Trans on the last change. I'll stay with the Amsoil (most likely permanently) and the Formula + (at least for now). I’ve been thinking about trying the Mobil 75W90 in the trans down the road.

 

Not knowing 1/2 of what the rest of you seem to know about oil, I follow what the manual says. And the manual on my sportster says either HD gear oil in the primary/trans, or HD synth in both the engine and primary/transmission.

I used HD synth in both for the first 25K miles, and just switched to mobil vtwin 20w50 in both.

Seems closest to what the manual recommends. If anyone knows why this is bad, I'm open to learning about it.

 

thae 75W90 is recommended by harley, the manual calls for 15w50. change it out and you'll see the difference. also, whenever you change to synthetic oil, you'll also have to buy a high performance oil dipstick. Reason for the high performance dipstick is so it can transperliforate the viscosity of the velcocity of the oil and not allow the velocity of the viscosity to dissapate into the engine. Very important.

 

The oil itself. Think about it. Every stroke vaporizes a tiny bit of oil. Some oilsevaporate slower than others but they all do somewhat. Thinner oils will evaporate quicker.
Honestly the great oil debate cracks me up. All oil will protect your engine. Conventional oil breaks down faster because of the insane temps an air cooled motor sees. If your using conventional in the summer 3000 miles is as far as you want to push it and if your doing slot of short trips even 2500 would be better. Synthetic lasts longer because it neutralizes the combustion by-products and shears slower.
Remember long before synthetic came around guys bikes were running off regular oil so maintenance is the key. I change out conventional once it gets black. It's cheap and helps me sleep. Synthetic in my bike goes for 5000ish miles. I don't sweat it because it is a superior product and can stand up for that long.
In the end guys use whatever makes you feel good and change it when you feel like it.