plan for in-car valve stem seals replacement

[xHeart]

5w-30 is too thin for these cars.
Switch to 5w-40 and oil consumption will reduce for more than 50%.

Seriously, would it work in extreme cold temp -- overnight street parking etc.
Synthetic or Conventional?

[misha]

Keep with 5 but move forward to 40 instead of 30.
That said.....5w-40 will act the same in cold starts like 5w-30,but will be thicker at operating temperatures than 30.
Feel free to use 0w-40 also.It will just be thinner at cold starts.Cold starts will be more easier with 0w than 5w but at operating temperature will act the same as 5w-40.

Fully synthetic,of course.