I just want to add my opinion in here. I have tried the oils mentioned above.. Castrol 0-30 ( GC), as well as the others, however I have not tried the Mobol 0-40, all that said.. you will be hard pressed beating the Shell Rotella T 5w-40, it's an outstanding oil that holds up very well.. even compares to the "synthetics" maybe evening beatings several of them in the long run OCI. I have never had any cold start problems with Shell Rotella T 5w-40, and I have used it in Honda 4 bangers, Cummins 5.9 Turbo Diesel, and many many more. I would not use any additives in any of the oils you choose, if you wish to try another oil, by all means it's the spice of life for sure, however don't rule out the Rotella T being at the top of your oils listed to try