Originally Posted By: Alex_V
I've owned an '85 C3500, 454/TH400 with 4.10 gears, for 10 years and 50,000 miles (77K to its current 128K) and use Schaeffer's 7000 10w30, a dual-rated, SN syn blend. If your BBC's have close to stock cams in them have no fear of "low" ZDDP - your syn blend Rotella 10/30 is a good choice.
If anything, questions about "what oil" will get more variety of answers with these old engines than any new 0w20-spec Pilot or Chevy 5.3. My opinion, regarding pre-port-injected Chevy V8's and inline 6's, is that 5w30 is only for fresh, tight engines that don't see extensive truck-type use. 10w30 or '40 is good unless it's run long enough, or is of a type (conventional) that's going to shear down - hence why I use Schaeffer's that never shows any oil pressure drop even up to last year's 6,400 mile OCI. 5w40 sets about the same as 10/30, and 15w40 isn't necessary unless you're working it real hard most of the time, particularly in a warmer climate, or the engine has some wear that needs a little thicker oil to keep it quiet and oil pressure up. Anything thicker than 15w40 is only appropriate for stuff like racing engines, with the appropriate knowledge to know what to use and why, or as a last resort to get a few more miles out of an engine that's, basically, completely worn out.
The "10w30 is too thin" crowd, I think, is largely those that haven't moved past the days when all but the finest oils sheared substantially in even these engines that aren't overly hard on oil. Today's oil chemistry is light years ahead of what our engines survived hundreds of thousands of miles on back in their day and the viscosity of a 10w30 HDMO will take thousands of miles longer to shear down to the same point that a 1980's 10/30 would have in 3K. Synthetic, "low" zinc, 30 or 40 weight - have no fear.
It's not about shear, it's about BBCs being sort of unhappy engines with sustained high rpm. In the marine industry, the idea of loading up a 30 weight into a BBC is almost hilarious. The amount of metal they shed in a short period of time is actually kind of astonishing.
Universal average for 50 hour OCI is about 50-70 ppm depending on the lab. We've gotten that down to the 20's on 100+ hour OCI's using Schaeffer and VR1 20w50.
Best UOA we got using a sub-50 oil was using RT6, delivering high 40s on iron over a 100 hour OCI.
Volvo Penta had the bright idea to use SAE 30 synthetix diesel oil in big blocks (much stouter than any passenger 10-30), and had engine failures inside of the 2-year warranty period. They then modified the spec to 15-40 diesel oil. We have seen a lot of VP 7.4 and 8.1 engines with awful compression 300-400 hours into service with documentation of regular service intervals on VP SAE 30.
Mind you, all of the above concerns roller cam BBCS with broomstick truck cams.
Point is, if you're giving a BBC average duty, a 30 is fine. Millions of trucks did it for tons of miles. If you're beating on a BBC and want it to last, go thicker or you might pay the price.
Where shear is concerned, these engines will eat all but the best oils for breakfast under sustained high rpm.