i am currently using Shell Rotella 15-40 as well. its good stuff for small engines and works well in warmer climates. i have used synthetic before, also. Mobil 1, 10w40. no issues with either and people swear by both. tbh i don't think it matters much (syn or dino oil) if you change it at correct intervals. the Rotella is dirt cheap, however. often on sale at Walmart for under $15 a gallon. i got my gallon for $11 at the local auto parts. it also comes in synthetic for a few dollars more but its a different weight
if you check the screen just be careful removing the "cover." i might even advise to have a back up screen cover on hand just in case, and now might be the time to get something magnetic, anyway. like this:
http://www.scooternerds.com/eshopprod_cat_6078-67311_prodtree_349-235-307_product_874494.Prima_Magnetic_Oil_Drain_Bolt_.htmthat piece on top is the screen cover. myself and others have broke / stripped it while taking it off. it is very thin, soft metal. almost pewter like. the threads or not machined around anything solid. so its more like an inverted jar like design. or something like that. look at this pic:
http://www.key-ideas.com/Magnetic-Filters.jpgyou see the cover upside down there? the other side has a bolt head design in the center for your wrench to grip, but as you can see its not a bolt! its hollow in there. so if its a struggle to take off be careful and go slow! i understand these things are made to strip and break easier then the case. and thats a good thing, as its much easier and cheaper to replace bolts and things then engine cases. but that just goes to show stripping and breaking does occur. so having an extra around is a good idea imo. if the screen has never been removed it might be "factory tight" as well. thats how i broke mine. sucker wouldn't budge and bam....stripped and broken!
check the dipstick after pouring in 600ml and go from there. "topped off" should be 800ml but you don't have to use that much, as even 600ml is 3/4 full and shows well on the dipstick. don't overfill! if you remove the screen cover you don't have to remove the drain bolt at the side of the oil pan. all oil can drain from the screen cover hole. you may want to tip the scoot in that direction to drain more oil out, and give the kick start a few....kicks to purge even more.
do yourself a favor and before warming up the scooter to thin the oil (just ride it for several mins) loosen the bolt(s) FIRST. then retighten until snug but not to specs. if you loosen the bolts when the engine is warm / hot the metal can expand and / or soften. not good if trying not to break anything. so loosening first when all is cold can make it less likely to happen
oh, and if you remove the screen to clean it just blow in it, wipe it with a paper towel (don't leave debris, of course), blow some air into it. but chances are it will be clean, anyway. never know, though. and replace the screen and especially the inner spring which goes with, correctly. you should feel some "spring resistance" when pushing the assembly back in, and all should line up well so you can properly tighten the cover
if you decide not to check the screen just remove the oil drain bolt from the side of the oil pan (like you do your car) drain, replace bolt, refill.