Again.. without knowing the brand it'* hard to say it it'* better then the other.
Synthetic isn't thinner then dino oil.. just more durable. ( as stated y 95naSTA )
Mobil 1 is a good safe bet. You can always look for German made quaker State.. both have done well in UOA ( Used oil Analysist )
If you're in Canada, it'* Esso XD-3 synthetic 0w-30 for me.
(i.e. 10W-30 )
the beginning is for cold start 10W
and the 30 is for operating temp 100C reading of the viscocity.
Tru as 90snSTA stated also that a lower viscocity 10w-20
at operating temp will yield a bit more power cause in a sense it'* thinner, but the chance of having your engine seize up due to lack of protection IMO isn't worth it. It might be ok for an emergency situation but not prolong duration IMO
You might also want to consider oil thats gasoline and diesel safe.
Diesel engine oil generally has higher detergent level then standard oil
Your missing oil could just be as simple as oil consumption that is burnt off in the combustion chamber so you don't see it ..
Opensourceguy has mentioned a trick that I will pass on.. There'* special UV pallette yo can get and in this case put into your engine oil.. run it for a while, if there'* external leak.. when you shine the UV light on it, it will glow right up and show you where it'* coming out of
If you don't want to list the brand here.. check out www.bobistheoilguy.com