hmm... mixing? for me ... better not ... u know ... afraid of
some chemical reaction... if I use esso 2t then after want to
finish, I change to vsx then I think it is fine (cos not many
chemical reaction will happen), but imagine : 50% VSX + 50% ESSO
... I dun dare to try ... Panadol is to cure headache, coca cola is
nice... but try to eat panadol then drink the coke... rumors say,
it is the fastest way to suicide... why? ... chemical reaction ...
If petrol, i think it is fine cos 95%+ of petrol from any brand
uses the same benzyna (i dunno how to write in English, I know it
is bensana in Indonesian, kind of Alkana, Alkuna, Alcohol, weak
acid) So even though 50% esso petrol + 50% shell petrol, that will
be fine for still thr 95% of that is benzyna.
Think about 2T, one got fully synthetic, one not fully, half, or
dunno what. Noone know the base chemical substances. You mix them,
there are some probabilities:
- The reaction give residue, it is like "salt" or "salt rock", then
if it hantam your piston.. go overhaul.
- The reaction weaken one another. Have heard about people cures
poison with poison? Same here, originally 2t, u mix ... then not 2t
anymore ... (neutralized)
- Unexpected chemical substances, and the "gas" it produce can give
toxin ... dangerous ... but it happened in places which doing oil
refinery. The gas is poisonous cos mixed chemical here and
there.
- Nothing happened ... and the bike is just like that ...
- Or your dream ... the bike become faster ... (I doubt
though)
Maybe someone ask, then why everyday we eat orange, banana, rice,
veggie, but got no chemical reaction? actually it happened ... but
our body can handle them properly (with kind of enzym) ... and ...
the temperature is not very hot... if your stomach is > 100
celcius ... then you may give poisonous residue (shit) or poisonous
fart ... 
just my thought 