hi sir Jackson!^__^
I'm sorry to hear that you've experienced such a very rare case of the strada emitting black smoke....

...and to think that it also happened 2 times in a row...
Unfortunately, from me and a most of the guys here...we couldn't give much personal advise with regards with your situation...because like what I said earlier...its like only once in a very rare time that we would encounter cases like yours....
However, I can give you some things to check onto & some hypothesis so that you can somehow point things out to your SA when you visit the dealership...
Here I go..=D
1.)
the one you said with regards to the OIL viscosity could also be the culprit.
but although you said that it was casa maintained and we assume that there is a very little room for error...I still wouldn't be so sure about it especially at times like what you are experiencing right now.....So please do some double checking on what oil was used and if its within the standard for the 4d56 and 4m41...
2.)
It might be having symptoms of a premature wear of the Suction Control Valve for the reason that you might have loaded some dirty fuel on it...and it just so happen that the real problems has not manifested yet because the dirt might not be fully accumulated yet enough to cause some changes in the engines performance and it turned out that the black smoke is only the first warning...so better have it checked immediately and not wait for the real problems to occur...
because there is already quite a number of TSCP members and owners that already experienced the SCV failure because of the dirty fuel....and having this black smoke is one of its symptoms and that doesn't exempt the relatively new ones if you really do have dirty fuel in the tank...
3.)
Since you've mentioned that it still only has 6,000Kms on the odometer I assume that it already had its 5,000kms PMS right? so for that reason the mechanic does the SOP of hooking its system up with the MUT for it to be check by the computer....So I suspect that the mechanic did some mis adjustments on the system which I think is enough already for it to produce that certain black smoke....because even though the mechanics had all the trainings necessary for them to effectively use this new system they are still getting jumbled up and confused at times because IMHO having a laptop infront of you and a wrench & screw driver at the same time in both hands still needs a lot of getting used to for our Filipino mechanics....But this is only a hypothesis there is still a big percentage that I could be wrong and it could be still something else...
So I would greatly advice that you visit your dealership the soonest possible time and I hope they can figure things out....

and please do update us after your visit...
HTH....^__^
and let's also wait for the others on what they have to say about your problem... to help you even more on solving the smoke problem...=D