This one is probably for the real seasoned old school mechanics. The owner's manual and service manual for my '79 Catalina calls for either 10W-30 or 10W-40 based on my region's temperature conditions. I've been using 10W-30 since I bought the car last year. I haven't fully figured out where it's leaking oil from, and I thought changing to a higher weight might slow it down a little, until I feel like actually looking for it. Will there be much of a difference between the two? Any other reason to use one over the other?
Going to a higher viscosity oil (10w40) is not going to solve your leak problem, and probably won't slow it down either.
My understanding from a friend is the slightly more viscous oil won't flow out of the leak as quickly as the thinner oil. If it's the rear main and not something above, the leak isn't bad enough to warrant being replaced, not yet at least. Oil is much cheaper than lifting a motor off its mounts, pulling a pan off and replacing that seal. I'm not even sure if the guys in garages nowadays would even know how to replace a rope seal rear main, as you know, that's pretty outdated design. Haha
Generally, you don't have to lift the engine off it's mounts. Pull the trans and rear main seal, replace it, and put it back together.
On the 10/30 or 10/40 the 30 and 40 denote the oil's viscosity when at operating temperature. There isn't much difference between the two but 10/30 actually in my experience gives better lubrication. Think of the numbers as being the size of the molecules, 40 being larger than 30.
To find the leak, mix some U.V. dye with the oil, run the engine awhile , then hit the engine with a black light. The dye will glow at the point of exit.
I've done the dye test. It's either the rear main or somewhere above, and replacing the rear main seal requires pulling the engine off the mounts, all of it is original (AT-205 will not restore rope seals, which is what that seal is in this engine). Leak isn't bad enough to warrant that kind of work. I know what the weights mean, being that this engine is old, I don't know why they would specify two weights, and under which conditions to use which. Couldn't be an availability thing even in the 1970s. 10W-30 is the most common engine oil weight.
Back in the day when your car was designed and built, people used heavier weight oil in Summer/the South and 10W-30 in the Winter/the Dakotas. Back then, engines didn't have variable valve train phasers and oil-pressure operated cam chain tensioners and such which make changing oil weight problematical. I know people with oil leaks and old engines who used 20W-50 in the summer, or added STP, which increased oil viscosity dramatically.
I thought that might be why they specified both. There's also a little valve chatter, nothing abnormal, so I thought the slightly heavier oil might reduce that. I do a lot of higher speed driving (55-60 on the highway) and thought the slightly more viscous oil would also help lubricate the engine a bit better, there's no overdrive. Sounds like I've overthought this one. Haha.
Those older engines can take small changes like that in stride.
I don't think the additional 10 viscosity points will slow down the leak significantly though.
Most of the leaking is probably happening while the engine sits cold (metal shrinks, gaps open). And both those oils are the same (10 weight) when cold.
Engines of that vintage are not that picky. I use either 10W30 or 10W40 in all of my vehicles depending on what I can get cheaper and easier. (Sometimes one will be in stock when the other isn't when I go to Wally World.) It's made no difference at all in the way they run.
The oil leak you'll have to track down. Neither of those weights will make a difference there. A 20W50 oil might, depending on the nature of the leak but you'd only want to run oil that thick in the summer. More typically those kind of heavier oils would be used back in the day to reduce oil burning a bit in a worn engine.
Makes sense, this engine doesn't burn much oil as far as I can tell, so the leak seems to have developed "recently". It'll come down in large drips, but not always. Oil gets on the hot catalytic converter on the passenger side and smokes for a few minutes after you turn it off, especially after a decent drive. If you remember, I was thinking it might be the rope seal rear main from where the dye was coming out. I dug into my service manual and noticed that the engine needs to be lifted to get the oil pan off of a Pontiac V8 in the B-Body of that era, because there's an inconvenient crossmember underneath the motor. That's probably a relatively expensive shop job. Not to mention the original motor mounts may disintegrate after being unloaded.
A friend said those old Pontiac V8s can leak from the intake manifold at the rear. Other than that, and the distributor drive, I can't think of another possible leak source. The valve covers are dry.
I'm surprised a '79 Pontiac engine is using a rope seal, by 1966 even AMC V8s had rubber/neoprene crank seals, albeit a two-piece job in the rear. (The earlier series V8 in my Marlin uses a "hemp" rear seal as called out in the shop manual.)
I am as well. Seems like an archaic design, but that's the only thing that will come up on every site I've looked at, even the service manual specifically describes how to trim and install the rope seal. GM must never have updated division engines, I'm pretty sure the Pontiac V8 went out of production in 1981. It was illegal for sale in California when brand new in my car. Haha. https://www.autozone.com/internal-engine/rear-main-seal/p/mahle-original-rear-main-seals-jv708/147431_0_0