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I'm currently using Royal Purple, 20-50, per recommendation from Butler Performance. What oil are you using?

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Brad Penn 20W50 currently but going to try their 10W40. The 20W50 is a partial synthetic but still seems too thick at startup, oil pressure gauge shows 85 psi even during the summer.
 

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Amsoil 15/40 for me.
 

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The only thing I have run 15/40 in was my dmax diesel. I have read lots of publications on oil, along with 10 years of circle track racing, and would never run anything more than 10/30 or 10/40 in a typical gas engine. Maybe include some addative for the flat tappet cam shafts.

I am curious as to what the rationale is for running such heavy oil??
 

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The rationale is that it's what is commonly available in a cheap, fairly high content ZDDP oil. 15/40 is not ideal, but it's certainly not too heavy in my neck of the woods, where winters are fairly mild and summers are scorchers. The owners manual calls f0r 10w-30, and in my experience with everything automotive, 10/30 is probably the best 80% of the time...until the temps get into the mid 90's. The difference between 10/40 and 15/40 is negligible, in my opinion. I have run diesel spec oil for years, and am sold on its additive package and anti-wear and anti-scuff properties. The proof is in the teardown and inspection, and it comes thru with flying colors every time. Can't say that about low ZDDP regular automotive oils for these old cars. The difference in scuffing and gouging damage in high load conditions between Rotella or Delo and regular oil is a big one. Brad Penn would probably be the best choice, but I can get a gallon of Rotella for under $20. And Valvoline Racing oil has little or no ZDDP, and has a race-formulated additive package that is unsuitable for the street.
 
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