Oil weights.............
In my automobiles I use  15-40 I think, perhaps 15-45.

In the bike engine 15-50
transmission and rear end 75-90 gear lube..........
All Mobile # 1 products. 
You may see an rpm increase a little at idle speed but nothing to really concern yourself about.

Another thing to remember is this. Most users of regular motor oil will change oil at a frequency of about every 3000 miles which means if you are off on an extended trip of 5 or 6 thousand miles you will have an to change oil somewhere along the way. With synthetics you can run .5 ,6  7 even 10 thousand miles without a problem....!!!   If you burn a little oil and are running low you can add conventional motor oil to your synthetic without any problem

In the research I have done on synthetic motor oil I have seen engines torn down after running the equivalent of 100,000 miles on the same oil with virtually no significant wear  whatsoever. These tests of course were run on a dynamometer  [ sp ? ]  however.  But after inspections there was also no sludge build up at all and the inside of the engines were as clean as you would want.

Regardless of what the test shows those of us who are obsessive compulsive still try and change oil at every 3000 miles regardless of what the tests show or what kind of motor oil we use.

Clarence
Pinehurst, NC.