Have a read through the Engine Oil Bible here
http://www.chris-longhurst.com/carbibles/
it's very readable, in straightforward language, and the guy knows his stuff. It will dispel some of the myths around about oils and hopefully answer a lot of your questions.
About your particular case, don't worry, a top up with synthetic isn't likely to cause any problems, it will mix OK with semi-synth (generally speaking anyway). It's nice in principle not to mix oils of different specs, but it's not a disaster.
One thing to be aware of is the issue of "bike oils", the main aspect of these is that they are specifically tested to be suitable for use with "wet clutches", which is what most bikes use (the clutch is inside the engine and runs exposed to the oil, unlike in a car where the clutch is in air and doesn't see oil). This wet clutch test is indicated by the term "JASO-MA" somewhere on the label.
The main advantages of fully synthetic oils is the high temperature stability and the ability to provide very long service intervals as specified for many cars nowadays.
The SV is perfectly happy with a semi-synthetic (I use Castrol GPS, but others have their own preferences).
Re. Mobil 1, I worked with someone who previously worked for an independent Oil laboratory, and he recommended Mobil 1 as being one of the best oils available. I use it in my Yaris.