Synthetics have been proven time-and-again to resist thermal breakdown way longer, to produce better mileage and better power than petroleum (dinosaur-juice). Anything dino-juice can do synthetics can do better longer. I've been using them (Amsoil, Royal Purple, Redline, Mobil 1, and others) wherever I can, whenever I can since 1985. Motorcycles, ATVs, snowmobiles, boats, cars, trucks, motors, tranny's (manual and auto), rear-ends, transfer cases, you name it, can all benefit from switching over to synthetics after initial break-in period. The motor, tranny, rear-end, etc. need to wear-off the high spots and smooth things out just a little bit to break-in. The sheer and heat during break-in is VERY harsh on all the oils. Whenever possible change out all the lubricants at the earliest recommended service interval. H-D recommends 1,000 miles. I'd always done the same with cars, trucks, etc for as long as I can remember. If you can afford it switch to a good synthetic product. Engines with 200,000 miles run like new and show no wear on inspection! Auto trannys really benefit from switch to synthetics.
I switched from factory dino juice to Syn3 all the way around in my
'06 Road King Custom (88 cu - 5-spd) at 1,000 miles. Tranny seemed noisy/clunky! I did some research and switched to Redline Syn 20W-50, Redline Syn Primary case oil and Redline Syn shockproof tranny oil. I switched to Redline in my
'08 Road King (96ci - 6-spd) at 1,000 miles. Just did the same with my
'09 CVO Road Glide at 2,500 miles. I bought it used with 1950 miles. It was serviced at Performance H-D in Syracuse at 1,000 miles for the 1st owner. He had some kind of gear oil in the tranny -- prob 90 weight, or so.
Redline syn tranny oil with shockproof was a major improvement over everything I've used. Redline primary case oil looks and acts very similar (same viscosity, etc.) to the way ATF does. My friends who race use synthetic ATF in chaincases on snowmobiles and motorcycles.
I like Redline products. They are specifically designed to do the best at each function.
I buy them at Hillside Motorcycle in Munnsville, NY. check out their website
www.hillsidecycle.com/