I see walmart has both 30wt and 15-40wt Rotella.
From reading up on it in past threads, it seems like 15-40wt may have just lessened the amount of zddp.
If there is CI-4 on the bottle, should I stock up on the 15-40wt?

Looks like 30wt has ok zinc levels at present.

I'm trying to determine what weight to start stockpiling.

I know 30wt was norm back in the day these engines were made.

 

i've been running my flatty on the 15w40 with absolutely no issues . and mine is a untouched original as far as i know , not a rebuild . if i can remember the zinc maybe was needed for the overhead valve engines as they have a much higher valvespring pressures and they needed it for lubrication or ???? anyway it works for me and mines as clean as a whistle inside . biggest thing i got from everyone was { my first flatty ya know } to change the oil without fault . i change mine every 1,000 period . or as much as i drove it when she's together about once a month ...................

 

High zinc levels in oil are needed on flatheads as well.

No timing belts-they have timing chains. The zinc helps the metal to metal contacts on the flatheads (like the timing chain).

on freshly rebuilt flatheads it's REALLY needed. I'm running lucas break in oil additive as well right now.

Zinc also prevents internal corrosion for these older engines that are not run all the time

It just destroys catalytic convertors, which will not be in my truck as long as I'm breathing

 

timing chains ???? all the flatties i've seen are gear to gear and the one in my truck is too . and it has one of them consarned nylon top gears which had me stumped till ross told me they were original .welp if they need zinc i geuss i'll be looking for some additive myself ! oh and mine when running is just flat driven and used and abused as stu and tim can tell you !!! mine is as they designed them to be .... driven ! daily !

 

I guess the zinc additive is just a myth for the old flathead engine then.
I'll be draining my oil and not worry about zinc protection myth anymore.

 

Flatheads have low valve spring rates, no pushrods, heavy hydraulic lifters, or rockers to throw around. Stock flathead cams don't generate high point loads on the high point of the cam. Zinc is a good idea for break-in of a new cam and/or lifters, but on a broken in flathead there doesn't seem to be any big risk to regular modern oil.

 

thats what i thought i remembered . getting old or goin' blonde not sure so the memory just isn't what it used to be ..............................