I've run Amsoil Synthetics in all three holes in my last 4 HD's and have no complaints. 20/50 in the engine, 10/40 and 75/90 gear lube. It doesn't leak or burn but I do trade or sell my Bikes between 10 and 25K miles.

Synthetics all the way,period.

I'm getting in this disscusion a little late and have read the previous postings all with good info, the question I have is , can you run the stuff in older evos? I've gotten conflicting info, the most disturbing is that it is so slick that your bearings won't spin correctly.

I have never heard of any problems running the syn in the Evos. As a matter of fact the following statement is on the Amsoil site....

According to Harley-Davidson, one of the unique benefits offered by Screamin' Eagle Synthetic Motorcycle Oil is its versatility. For the first time the company is recommending and oil that can be used in the engines, primary chain cases and transmission of most Harley-Davidson motorcycles, including Evolution XL, Evolution 1340, Twin Cam 88 and 88B, Revolution and Buell models. This feature, however, is not unique. AMSOIL 20W-50W Synthetic Motorcycle Oil may also be used in the engines. Primary chaincases and transmission of these applications, providing superior lubrication protection in each component.

I'm not a mechanic and don't play one on tv.

I've always used Royal Purple. Because it works, and it makes it easy to figure out which one goes in which hole. You can use it in all three hole. 20W50

YMMV

I have been a certified harley tech for 10 years now and I can tell you it all realy depends on the aplication and outside temp. As for the whole should I run Synthetic I can only really see doing it for fluids that don't get changed much. even then it is still not worth the cost. If you maintain any vehical at its recomended intervels or more your not going to have an issue.  evo motors were desinged to go 80K plus before needing a remaufature to bring back to stock specs.  Twin cams are 100K plus be reman.  If you are beating on your bike: cold starts, excessive reving,  burn outs, no clutch shifting yea you want sythtic. It sticks to metal parts or metal parts can absorb it a little. 
In other words if you are nice and care for you ride  you dont really need synthtic. If you beat the crap out of it yes you do.

Full Syn. Rotella T Syn in everything I own.

Amsoil all three holes

I bought my Suzuki Boulevard C90 new. Did the breakin miles and the First service. Used Amsoil in the Motor/Transmission and also the shaft drive gear. The Motor ran cooler, Clutch Gear was quieter, shaft gear cooler, and milage increased 1.2 mpg over next 1200 miles. Oil cooled ran nice in an escourted parade that walked near a half mile while some others were smokin. I will stay with it

Lots of good info except what Synthetic oil is. It's manufactured in a lab.

Say you take one drop of Dino and one drop of Syn, look at them under a microscope, the Syn molecules will all be the same diameter, say 1/2 inch balls. The Dino molecules will vary from 0-1 inch diameter balls, this is what makes Syn oil lube better, it's consistant size. Engineers this past decade have used this fact to create tighter tolerance designing engines, thats why you see factory recommended oil viscositys at 5-20 and 10-30 for most newer engines recomending Synthetic oils. Many new vehicles come new with it in the engines.

I have researched this topic to death the past few years and found equal arguments on both sides of the fence, some say Synthetic oil is too slippery for new engines, it won't allow rings to seat in properly and say wait a couple thousand miles before using it. Others disagree, it's a fact some new motors come with it already. Personally I use recomended Honda standard 10-30 and change it every 1500 miles, there's no substitution for clean oil. The reason I change it so often is my engine spins 10000 rpms and I run the shit out of it on occasion. If I had slower engine I would go more miles on the oil.

Some want to use thicker oil, 20-50 instead of recomended 30 or 40 weight, my feelings are this. The viscosity is determined by engine clearance, main bearings, piston / cylinder clearance, ect ect. Say the factory recomends 40 weight oil and you put fifty weight in it. Try a little experiment, set a spark plug gap at .040, now cram a .050 feeler guage into the gap........... thats what you are doing using a thicker oil, you CAN cram it in there but you are forcing it  causing more friction to do so.............

I use synthetic oil in my 2010 street glide. so far so good!!

Use conventional oil until your bike is broken in, then switch to Amsoil Synthetic Oil.

This is a rather lengthy article but addresses the difference between synthetic and "dino", breaking in with synthetic and viscosity.  The author recommends synthetic oil for unit constructed engines, like the Sportster or Big Four bikes and feels you get the most bang for your buck with Rotella T Full Synthetic.

All About Motor Oil (LINK)

Yes,,, The author is correct that Rotella T Full Synthetic is good and readily available. IF YOU CAN NOT GET AMSOIL, then you should buy Rotella Fulll Synthetics until you locate your AMSOIL SYNTHETICS Motorcycle Oil. Amsoil is THE BEST synthetic oil out there. Follow this link and read comparison test: 

http://www.lubedealer.com/embry-synthetics

What syn would be an improvement for a wet clutch in a Honda 4 stroke motor? Been running the Honda oil w/out moly since I purchased the bike,per the manufacturer instructions.Change the oil at 3-5000 miles,yet I have never noticed any metal shavings? Do you use magnification to see the shavings?
TIA

What oil are you running now, What year a model is the bike ??

Well, I always used synthetics lubes. When I was younger in the 80's I learned of the benifits while racing cars and motorcycles. I must admit I started with Mobile1 for 2 years but later learned on Amsoil and have used nothing but Amsoil ever since. Cars, Boats, Airpalnes and Motorcycles all have Amsoil synthectic oil and Amsoil filters.
Most Trucking companies use Amsoil products, taxi companies are using it, etc..... Less Maintance.....

AMSOIL oils work great!! We use it in our 1000hp drag car, tow rig, golf cart, passenger car. Just Plain works

Syn Motor Oils have smaller molecular structure then Dina Oil. This smaller structure help to prevent friction but will not be as good to seat in your rings and pistons and create the clearances the engine was designed to ultimately run with, so for the first coupe hundred miles run Dino Oil, in some cases that specially for Japanese, Italian and German bIkes the recommendation is not to switch until after the first regular Oil change or 3K miles, some will recommend 10K Miles. Smaller Molecular structure = less friction = less heat created. I had mentioned that before Gasoline post - Ethanol is a degreaser and will over time work its way into your Motor Oil and dilute it, so keeping reasonable Oil change Intervals is critical. There are several places out there you can send your Oil to that will test it for a few bucks and give you the current status if you want to go through that to determine the optimum Oil change times for your bike. Also depends how much you ride your Bike - Oil and Ethanol will change acidity of your Oil and if you engine sits for long times, it will be harmful to your Engine parts and Seals, so even if you bike does not run but lots of sitting around - like over the winter, before you take it out, or shortly thereafter, change the Oil.

Motor Oil today also has a special chemistry section for Silica, Moly, Sulfides and Zinc derivatives that help a lot with Lubrication. Royal Purple and some others have even special high friction oils like the EXP series for example that contain much higher then what is now considered normal amounts. To much Zinc will also create problems with metal shavings in your engine over time, so if you use this kind of oil carefully check your hopefully magnetic plugs and oil filter caps. I use both - both magnetic. There are charts available for chemical contents of Motor Oils and they vary a great deal. Some now have reduced additives to a point where I would no longer consider them recommendable - so if you really want to check things out that is a good way to go. But in general a good known brand of Syn oil will be fine. Those additives are also the parts that make it bad if you have wet clutch like a Motorcycle, Moly and other small molecular friction modifiers especially will kill your clutch plates in a hurry, so they can be used in Dry Clutch (no Oil like most BMW Motorcycles and Cars) but not in any Wet Clutch situation, so always look for the approved Motorcycle Oil = less slippery additives. To high an additive amount will also kill your CAT = expensive to replace once you killed it - goes for Car and Bike. I run the highest grade additive Oil in my Transmissions and final drives - no issue with the CAT and best lowest heat generating structure - all of it synthetic - but stick with the grade the manufacturer recommends to not create any warranty issues. My 2 cents

Also going back to other posts - some manufacturers will manufacture for others under different names - Honda Oil has always been good oil but is also sent out and renamed or other brands or by other brands. Oil is an issue with many a book written about it and as many opinions as molecules in a BottleStick with a good oil, change it frequently, just like with many other things - treat your baby right and it will work out well.

The magnetic plugs I mentioned previously will do two things for you - they will tell you if you have any metal shaving off in the engine and depending on the amount be an early warning system for you by collecting it for you - to many shavings - something is up you may want to get checked out or need to get checked out. And by collecting those shavings, a magnetic plug prevents them free floating in your engine creating further issues by recycling them over and over through the system. I have magnetic plugs in all areas of my cars and Bikes and also on my oil filters.

i have a 98 f-150 (i know its not a bike) and it has over 250000 miles on it and all that has ever been put in it was synthetic motor oil in it same with the wifes van 160000+ on i use it in my vstar and my streetglide, but dont make the mistake my brother did and put car synthetic in ur bike u will end up put clutches in it in about a week............

One item I did not mention but it came up once again today - Dino Oil vs Syn Oil in older Bikes. If you have been running Dino OIl for a long time in your older bike, everything is settled in and all the seals are doing fine - using Syn Oil may due to the structure of Syn Oil cause some seeping / leakage on the older seals. Thought I mention it.

This thread is pretty old, but here's my $.02 I agree synthetic oils preform better. Petrusrock above is 100% correct. They did not work well on my XLH . In the engine, the top end sounded like an old typewriter, clickity clack... In the Trans, the clutch started to slip. Worst of all, oil seeped from the gaskets and seals.

I went back to a mineral based oil and all three issues were resolved.