First, why are you running such a heavy oil?  15W40 is a Diesel weight oil from my experience. 

Second, if you have leaks with conventional oil changing to synthetic will make the problem worse, not better. 

Toploader wrote:

I didn't think 15W40 was bad. At the last oil change I couldn't find any 10W30, so I went with the 15W40 instead. My climate doesn't get super cold and rarely dips below 40DegF, so I tend to look more at the second number and what the oil does at operating temperatures.
With the oil drips, I wondered about something with a higher second number to see if that would help.
I thought synthetic might be ok if I went thicker, but I will avoid it.
What oil should I be running on an engine of this age, with around 50,000 miles on it?

The 289 in my 66 has over 110k miles on it. I use Pennzoil High Milage non synthetic 10w30. It idles with 40 psi oil pressure and uses no oil between changes.

Toploader wrote:

With the oil drips, I wondered about something with a higher second number to see if that would help.

Not trying to be a jerk but the normal way of addressing a drip/leak is to find out where the fluid is coming from and fix whatever the issue is.   Usually it's a seal or gasket that's not too hard to replace. 
 

Toploader wrote:

I didn't think 15W40 was bad. At the last oil change I couldn't find any 10W30, so I went with the 15W40 instead. My climate doesn't get super cold and rarely dips below 40DegF, so I tend to look more at the second number and what the oil does at operating temperatures.
With the oil drips, I wondered about something with a higher second number to see if that would help.
I thought synthetic might be ok if I went thicker, but I will avoid it.
What oil should I be running on an engine of this age, with around 50,000 miles on it?

The oil these engines were designed to run was based on an antiquated grading system that's no longer in use.  The closest comparable product would be 10W30, but I run 5W30 and it keeps the oil pressure where I want it.  You don't need more than 10psi per 1,000RPM, so if it spins 6,000RPM it will be fine with 60psi of oil pressure.  Pumping thicker oil and jacking up the oil pressure just wastes power with zero benefit.  Its also working the oil pump harder and placing more strain on the oil pump driveshaft, distributor gear, cam gear, etc.
 

Thanks for the opinion on the oil additives. I suspected it was snake oil for many years, so I will avoid them.
Sounds like I live with it and eventually look into replacing the rear seal.
Oil wise, I'm tempted to try a 5W30. Would be interesting to see how that weight goes. Has to be better on start up at least.

KM wrote:

I ran with 10w40 regular oil with a zinc product added for the first 2200 miles the trip to Texas. After I got back drained and switched to Amsoil 10w40 Z rod full synthetic. 

I run Amsoil in a lot of stuff.  Its really good oil, but the way you did it is the right way to do it.  Break the engine in on conventional then switch.  I did that with my WRX (switched at 10,000 miles), and no issues until the typical Subaru leaks developed.  I redid the whole top end and resealed the engine at 195k miles and it doesn't leave a single drop.

I'm always a bit baffled by the weight of synthetic oils.  Temperature doesn't really seem to effect the pourability much.  The key I think is the grading is based on whatever test to which they subjected the oil, so if they are going for 10W40 those are the tests they do.  If it meets the tests it gets qualified as 10W40, but I bet it would also pass the test for 5W30.  Look at synthetic gear oil for example.  It pours like engine oil, but its still rated at 80W90.  I tend to just run whatever weight oil is spec'd in synthetic if I go that route, but I know there are claimer classes of racing using crate engines where they run 2 quarts of 0W20 in the engine to get every last fraction of a second they can.  If you do the math the oil pan is empty by half track, yet the engines last an entire season like this without damage.  Its really, really good oil. 
 

Well they did say we were going to run out of oil.  I just didn't think it would be like this...

Yeah, most vehicles have used synthetic oil from the factory since the mid 2000s, so the demand for old school oil is probably less.  I'm sure heavy equipment and tractor supply houses will always carry it because old equipment remains in service a very, very long time.  Marine places as well, but it will be 2x as much as at the auto parts store due to the equation of marine cost vs. automotive cost. 

I read an article several years ago basicly saying synthetic oil was real oil with more refining and more additives in them. Therefore the name synthetic. I went into more detail but you get the jist of it. You do need to be careful about changing from regular to synthetic because of the cleaning ability of it. It has the tendency to know out the big chunks of sludge and then it plugs the small ports and the engine then craters.

There are a lot of misunderstandings about synthetic oil. It is best to read about it. But the bottom line would be that synthetic oil is most different from refined mineral oil in that the synthetic oil base is a more homogeneous collection of properly sized hydrocarbon molecules while refined mineral oil contains a whole host of different sized hydrocarbon molecules including many molecules that are waxes (not good). One of the biggest benefits of synthetic oil is it's ability to flow when cold after being used several thousand miles. Used mineral oils have a really degraded cold flow rate after being used.

Example Article: https://www.machinerylubrication.com/synthetic-oil-31800

If you want to watch a lot of oil testing instead of watching TV, you might find these tests interesting:
https://www.youtube.com/c/ProjectFarm/search?query=synthetic%20oil

Thanks guys. It does look like I am stuck with the mineral based oils until I rebuild the engine.
So, is the 5W30 going to be ok with the zinc additive?

Last edited by Toploader (9/17/2021 3:48 PM)

BASIC INFO

https://www.amsoil.com/why/synthetic/

INTERESTING TEST INF

https://www.amsoil.com/performance-tests/