CJ2a Oil and oil pressure

[SteveBonny]

Hi all,

I was wondering what is the recommended engine oil to use in a 47 CJ2a?
I'm currently running a decent quality 15W40, I see about 30psi at idle on a hot summers day and about 60 psi at ~45 mph. Is this typical for a healthy engine, it was higher than I was expecting. Wondering if I should be running a lighter oil.
Thanks for any thoughts!

[LarrBeard]

The recommendation for the original L-134 engine was straight 30-weight, non-detergent.

Now, some rationale. The engine is a low-temperature engine. 160-degree thermostats were recommended for all except extreme northern climates. A multi-viscosity oil needs to get hot to get to its highest viscosity rating. Although an oil may be rated 15W-40, it may never get hot enough to make it to 40W. Yeah, 30W seems a bit heavy for cold starting, but its what was called out for normal operating temperatures.

Why non-detergent? Folks have found that even in engines with fairly low hours after rebuild, crud helps seal up the notoriously loose Willys oil seals - especially the rear main seal.

As for oil pressure, it seems a bit higher than most, but if the seals hold it - I would not complain or worry. 30 PSI at summer idle is really good.

And, the usual disclaimer - your results may vary .....

[gmwillys]

I would be overjoyed if our 46 had 30 psi at operating temp. 10 to 20 psi is about normal for a tired 134. 15w40 is a bit on the heavy side, but years ago, everything in the shop ran 15w40, because we had it in bulk. The 300 six in the shop truck was slow to turn over in the winter, but held good pressure when hot. The journal size on the 134 is large enough to handle the viscosity of the thicker oil, but on the F head 134, it might be hard to get enough oil to the exhaust valve rockers.

[SteveBonny]

Thanks for the advice. I hadn't thought of the detergent concern. The oil I'm using is likely loaded with detergents. I'll see if I can find some viscosity curves for straight 30 vs 15W40. I don't have the history for the engine in my CJ but it has been painted and I assume rebuilt at some point. I put about 30 more miles on it at the w/e and nothing broke or fell off and I made it home - so I'm feeling pretty good at the moment!

[LarrBeard]

... I'm feeling pretty good at the moment!

I love my old truck, but just wait - it's a setup! As someone wrote only last week - don't go anywhere you aren't willing to walk home from. Seriously - the more you drive it, the more confident you can be in its reliability. But, it's still an old vehicle, so don't be surprised when stuff happens.

[SteveBonny]

Haha, I think the walking home comment was me! I've started to carry tools with me now so that should reduce how often I break down!

I found some viscosity comparison numbers. Surprise to me was that the straight 30 has lower viscosity that the 15W40 at typical summer cranking temps. The multigrade must become effective at very cold temps, I didn't find clear comparison data on that.
As I don't have a roof or a heater, I'm not too worried about the winter temps.

Some numbers FYI-
2883