I was still in Northern BC when I noticed my fuel economy improving, so I am pretty sure it was Winter diesel.
As for the Child seats here is a pic of the LATCH weld job and bolts on the floor for the rear restraint belt. Of course, the welder said that he is not liable, but that he would trust it with his kids. Regardless, it is definitely better than what I had before. If you have this done, I would strongly recommend that you take in your child seat with you to ensure that it will work. It took a lot of research to find out dimensions etc. (sorry, I don't have the specific websites, google LATCH specs), but basically the diameter of the steel where the LATCH system connects is supposed to be 6mm, and placed just behind the bite of the seat where your seat belts are located.
Transport Canada told me not to do this as it has not undergone safety requirements (ie. I would have to send my van at 100kms an hour into a cement wall and see if it all holds up), or accredited by them.
***I am not suggesting for anyone else to do this. This is just to show what I had done, and by no means am I liable if you choose to do the same ***
Just the nature of the world we live in today.
Cheers