Oil Pressure, Weights and Changing Seasons

I have a 1990 Jeep Wrangler with the carbureted 258. I do most of the maintenance myself - from fluid changes to parts replacements, including swapping out the cruddy stock carb and upgrading the ignition, so on and so forth. So, I like to think I know my way around with a wrench, at least some of the time. But, I’ve learned mostly from manuals, the internet, and friendly advice, so there are large gaps in my automotive knowledge.

So, my question: the engine is old, but runs strong and smooth. My oil pressure at idle when warmed up is 20psi, and around 40 psi at 2200 RPM, or thereabouts. Does this sound normal?

My second question: I run 10W-30 oil. My friends father in law, who is a professional mechanic (owns a shop, not just a technician) suggests that I run 15W-40 during the warmer months (I live in Florida - it gets darn hot), and 10W-30 in the winter. Is this a good idea, and does anyone have any suggestions regarding this advice?

Given the age of the car and engine (113,000 miles, by the way, I’d like to make the best decisions possible to keep it running well as long as possible (it probably goes without saying that I don’t have the owners manual to check, and Hayned and Chilton’s aren’t very specific on this issue - including oil pressure). What say you?

It is quite common, even traditional, for those who live in hot climates to use a heavier weight in summer. It would be nice to know what the owner’s manual says; however your father-in-law’s advice is quite reasonable.

Since you live in Florida, you are fortunate that you can pick a wide variety of oils. Provided you don’t go visit you uncle in Vermont in January, you can pick pretty well any oil you want. We lived in a tropical country (75-95F all year) for 5 years and for cars the only oil sold was 20W50!

Your friend’s father-in-law has a point. In an older engine with generous clearances, and no cold start problems, go for the oil that allows the heaviest bearing loads; the oil used in Heavy Duty diesel trucks, which use 15W40 most of the time.

Modern oils are formulated to maximize gas mileage and also provide good cold weather starting, and minimum emissions. So we have 5W20, 5W30, 0W20, 0W30, etc. None of these is of any interest to you, unless you want to squeeze out every last mile out of a gallon.

In Florida 15W40 in the summer is great; it is the oil I recommemend to my trailer towing friends for their pickups.It has a pour point of -27F! So, don’t worry about it not providing lubrication on cold starts.

Thanks for the input guys. I think I’ll switch over to 15W40 at my next oil change in the Spring; probably Shell Rotella, as I’ve heard other Jeep owners say good things about it (I’ve been using Castrol GTX)

Any thoughts on the oil pressure - 20psi at idle and 40psi at speed?

The numbers don’t mean much. Such gauges can be wildly inaccurate in terms of actual values. All we really know is that your oil pressure increases with engine speed, which is what it is supposed to do. Your results sound OK to me.

20psi at idle in the hot weather in a hot engine is fine. You do not have a problem.

Having established that, you got here by following the owner’s manual recommendations. It has worked. I’d suggest continuing to do so.

www.carbibles.com has a good primer on the basics of oils. I recommend a visit.

Thanks for the link, it was very informative (as has been the rest of the information on this thread).

So realistically, even in the summer a slightly heavier weight oil like 15W20 really won’t make any kind of reasonable difference one way or the other, under normal driving circumstances, if I understand correctly. Although, a heavier weight oil may be useful under heavier use (such as towing)?

This makes sense to me logically…I think I’m on the right track!