Instead of using the usual Napa 15-40 all-fleet (Valvoline diesel oil) or Chevron Delo oil this change, I used regular 10-40 Napa (also Valvoline) with the usual Napa Gold 1064 (short) filter. For the past few months, on cold starts the engine would rattle some untill the oil pressure came up. Now, I have instant pressure after engine fires and 5psi higher pressure at cruising speeds (50psi) and idle in gear (30psi). I don't think I'm going to use diesel oil anymore. The only reason I did was because I read it had additives that where good for our old engines. On the other hand, oil today, no matter the rating, is still better than the oil available when our slants where new. Just thought I'd pass my experience along......

That's why I still don't use diesel oil, even though a lot of people recommend it. It's thicker, so obviously it's going to move slower until it reaches operating temps; not a good thing. The oil pump has to work harder and pressure will indeed be lower. As far as the additive issue (zinc), I would think that cam wear would be similar between using a thicker diesel oil using the additive (more wear on start-up), and a thinner oil (newer oils) which circulate through the engine much faster on start-up.

This is just a personal, not professional opinion, of course...

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Eric,

My wife's '72 Duster would have a little rod knock on cold start until it got oil pressure too. It actually sounds like the 10W-40 you have is heavier than the 15W-40 giving you higher oil pressure and is also cushioning the rod bearings. Either that or you finally found an oil filter that isn't draining back so you're getting oil pressure sooner.

Diesel rated oils are especially good for flat tappet cams and other sliding contact and that is due to the higher level of zinc. With a well broken in engine with a mild cam and soft valve springs it's not nearly as important to have the zinc. Regardless I would not use 10W-40 as it has the highest level of viscosity index improvers which are not oil, do not help to lubricate the engine, and tend to promote sludge and deposits.

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Joshua

I tried the 15W40 and had a similar experience. Slow pump up, and a rough idle cold. Changed the oil after a couple hundred miles, and since I had it anyway, went to 5w30 Dino-oil. It pumps up quickly, and seems to be doing much better. I've put about 600 miles on, and I'm not using any oil, so don't see that my engine really needs the thick stuff.
I'll be back to 10w30 next change.

CJ