While I too believe that 50W oil will cause some lubrication problems, I will respectively disagree with those saying it shouldn’t be tried in this case.
The OP has a 19 year old car that needed a bandaid so it could be driven for another year. The only other option was to fail the emissions test and take the car off the road.
I have seen many cars stop smoking by adding thick oil or several cans of STP-like products. For cases where a car is on its last legs and not worth expensive repairs, those options are quite reasonable to consider.
In the OP’s case, any inexpensive approach that works is better than taking the car off the road.