I've been assuming that seeing the spinning yellow circle is a proper sync. If I'm right, then entering and leaving an ammunation forces a sync. Not sure if you have to buy anything, but I buy some ammo before I go.
buying stuff doesnt always work, i bought some clothes, then when i came back my 80k apartment and the cash was gone but my new clothes were still there but i was back in my old garage...
and even after the patch when i booted yesterday i wasnt at the same RP and was getting texts about how things are now unlocked which were already unlocked and i participated in the day prior.
they need to revise the save system and make it more obvious when things happen, i remember on day 1 actually seeing a popup in the lower right corner if the servers werent availible, i then assumed when i dont see that popup with the circle, that its saving. that doesnt seem to be the case.
and like you said, i think the direction of the circle means download or upload but depending on the terrain and each of the map where that circle is, its sometimes not even visible or you're just looking at the cops/enemies shooting you to notice.
there should be an option to go into our garage or apartment and FORCE a save or something, not this "oh yah you can totally leave when ever and we'll remember where you left off, no really, dont worry about it, na we dont have to write it down, ill remember where you left off, k bye"
and then each day you come back "no you rank 17 right? yeah, did you have these missions unlocked? are you sure, well just to be sure and fair to everyone else im gonna take some RP away and in a few minutes you'll get a text about all those new fun stuff you can do, by the way have you seen our microtransactions section? mad about your money constantly going missing? we'll you can "speed up that process' by buying GTA bucks! only 9.99 but the choice is yours, and remember this online feature was a FREE addition to the game, arent we so nice, giving you a multiplayer feature in a game in 2013 included with the $60 price tag? pat us on the back cause its not like GTA IV, or RDR or Max Payne 3 had mutliplayer, so this was a surprise to everyone im sure" = 98 Metacritic.