If what you're using has worked fine for 19 years, why change it now ?
If it ain't broke, don't try to fix it.
I use Rotella T4 15w-40 in my 455 bracket engine. I've read that it still has a 1200 ZDDP level. Some of the diesel oils meet the SM rating, which have a max of 800. These have wrote on the bottle that they can be used in diesel & gas engines. I've read that this SM standard was established in 2004, to protect catalytic converters.
I realize that not everything you read online is correct.
At WM, I recently noticed that the Delo 400 & the Wal Mart house brand diesel oils had the SN or SM rating, stating that they can be used in gas engines. The Rotella T4 oil I bought does not include an SM rating.
https://www.walmart.com/ip/Super-Tec...7a&athena=true
https://www.walmart.com/ip/Shell-Rot...hoCM-IQAvD_BwE