I´m a newbie here so I thought I´d say hello, proud member of Caddyinfo too I might add.
I got some questions about oil...don´t wan´t to start another debate here.
My -93 STS requires 10W-30 oil API service grade SG, but since the current grade is SM I got a few questions.
Am I right about grade SM being better or at least being equal to grade SG with respect to wear? If that is the case, is it also ok to fill my -88 Eldo with it? My Eldo requires grade SF.
The reason I´m asking is because I´ve seen posts about filling 4.9 L with 15W-40 oil with a higher degree of ZDP (ZDDP) eg "diesel oil" to get better protection against wear for the distributor gearing etc.
Why is it necessary to do that if the grades are getting increasingly "better"?
Isn´t the backward compatibility in SM reaching down to SF, SG or what?
Please excuse my bad english.