My 409 doesn't like synthetic.

[Iowa 409 Guy]

Synthetic tends to run like horse-p1$$. Seeps through everything, which is why oil leaks sometimes develop.
I''d suspect that the engine got "rattly".

I put synthetic in the tranny on my old Knucklehead..............leaked like a sieve. Drained out what was left and put in 90 wt gear lube.........problem solved.....

 

[jwhotrod]

Im using Brad Penn 20w50 which is a semi synthetic, Pennsylvania grade crude base stock with synthetic viscosity package. The motor seems to like it fine.
Big Jim

 

[yellow wagon]

Brad penn is great stuff!! Lots of the racers around here use it. Bearings look like new when you tear it down after a season. I run the 10w 30 in mine.

 

[poison ivy]

shell rotella,20-50, 1/2 pt eos from chevy,4 speed, 4:11 gear , 6000 rpm every time i race the wind.

 

[Dennis Eudaley]

I use Mobil 1 15W50 in raced, but babied(no hi R's) 409's & have for probably 30 years. Yes, it does tend to find anywhere there are leak possibilities. Also use synthetic in rear. I think it precludes having to have extensive warm-up. BTW, raced my previous 409 for 15+ years w/o teardown on Mobil 1. It ran fine when removed, but had cracked block, oil pan & I wanted to go faster for NSS. Also BTW, entire NSS operation is for sale...see for sale section.