I asked about mixing 15w/50 with v-twin 20-50 in a bike that leaves about a quart of oil in the sump that can not be drained. And about the discrepancies of ZDDP and included a link to their own chart:
"I would still have to discourage mixing dissimilar SAE grades. They're not formulated with a mixture in mind, so I could not guarantee it would be safe.
I'm not sure who VOA is, but the 1200ppm and 1300ppm levels you noted in Mobil 1 15W-50 have been consistent for several years running. I couldn't be sure if someone else's chart is simply outdated, or was never accurate to begin with. Internal security protocols are going to prohibit me from following your link, sorry.
Thank you for choosing Mobil, "
And that is both the correct answer and it comes from an entity that would know. They should discourage mixing as there's no guarantee that any mixture would still meet the specifications or approvals that any individual oil carries.
Thanks for posting that. One thing that has been consistent on here is the inconsistency of Blackstone analyses.