I watched a neighbor change the motor oil on his old pickup last weekend . . . about a 77 or 78 dodge with a slant 6. He told me that his Summer “mixture” has always been 20w50 and a can of STP. I asked him why . . . he said it quiets the motor down and he has done this since new. With all the recent talk about valve lash adjustment, it makes me wonder. Wouldn’t it be better to adjust the valves on the motor (I think that the slant is a bit noisy myself) and use the proper weight oil . . . 10w40 or whatever Dodge suggests . . . couldn’t be 20w50 . . . could it? Anybody here ever used such a heavy oil mix? Wouldn’t this tend to starve some parts of the motor until it warmed up and flowed properly? He has done this for about 30 years . . . and although kinda rusty, the pickup runs pretty good. Rocketman
50 weight oil with STP added? That engine must have an incredible amount of wear on it! That mixture would flow like molasses, even at high temperatures.
In summer the 20/50 won’t hurt anything although my opinion is that 20/50 is a bit heavy for a vehicle that sees a lot of short hop trips; say 2-5 miles. The STP is unnecessary and of course 20/50 is too heavy for winter use.
I’ve used 20/50 exclusively during the summer months on both my old Mercury Sable and my last Subaru with never a problem.
Since I live in an outlying area most of my driving usually involves 20-25 miles at a minimum so the heavy oil is acceptable. The Sable had 420k miles on it (about 300k on 20/50) and the Subaru had 300k miles on it (about 200k on 20/50). Never any noise or oil related issues at all.
Should have added that I hope your neighbor is not trying to shut up noisy valve lifters by doing this. The older slant sixes used mechanical lifters that had a wide valve lash if I remember correctly. This does tend to make them noisier. He would be better off pulling the valve cover and inspecting the lash anyway.
He’s probably trying to reduce oil consumption on a worn engine. The last vehicles I had that used 20W50 were air cooled VWs/porsches. Does that engine have adjustable valves, I don’t know? I would use the recommended oil regardless of engine age, and follow the maintenance schedule.
It has always been traditional for those of us in southern climes to use a lighter weight oil in winter and a heavier weight in summer. This switch is especially recommended for cars of previous generations. If your neighbor has actually kept this baby since it was new, what more proof do you need?! We should all use his method!
It has always been traditional for the rest of us to read the owner’s manual and use the correct oil. (-;
Some engines do recommend different weight oils for different ambient temperatures, but synthetics have a very wide viscosity range the usually allows the use of single oil all year. For example, when my car was built 15W40 was recommended for high ambient temperatures and lighter oil was recommended for colder temperatures. Now I just use 5W40 synthetic all year (it covers the entire temperature range), much less hassle.
If he lives in South Florida or Hawaii, 20W50 oil is fine; it’s prettyy standard in tropical countries. But it will cause excessive engine wear on startup/warmup in colder regions. The STP would only work on a thoroughly worn engine! I used it in a very old stovebolt 6 Chevy during my college years.
In short, don’t follow his example, especially with a newer car.
We live in Pennsylvania . . . and it does get cold in the winter. I’ll ask him about winter oil . . . it has me curious now. I’m betting that he has inspected & adjusted the valve lash . . . he’s retired and spends a lot of time in his garage, but I gotta ask. I think that he told me that he’s been using this “mixture” since new. Gotta find out how many miles this old pickup ahs on it too. I’ll post back later. Rocketman
Well he is paying more for gas with that heavy oil and it will take longer to get the oil into the engine when it starts up, so he is also getting more engine wear. Even 30 years ago we knew better than that.
I had a 1964 Dodge Dart that had either 139,000 or 239,000 miles on the clock. Living in sunny southern California I always ran 20w-50 in the summer and 10w-40 in the winter. Oil loss was not too bad when the valve covers and intake valley seals were intact. On a run up I-5 to Bakersfield, I would get some oil blown up into the air cleaner. Leak down test indicated 40-50% leakage so the rings are well worn – the leaking air could be heard exiting the crankcase. There were no bearing problems during the duration. Oil pressure on a cold morning would turn out the OP light while cranking. If the engine didn’t start the pressure switch would not reclose for about 10 seconds. Checking the valve clearances had never indicated any oil starvation problems to the valve train.
Using 20w-50 is a good alternative for an well worn engine that originally used 10w-30 or straight 30 weight IMHO. If there are problems starting with straight 30 weight, the 20w reduces the starter drag for quick cranking and higher battery voltage for reliable starts.
Do you know what oil he’s running in the winter in Pennsylvania? Surely it’s not a real heavy oil because a container of 20/50 on a 25 degree day will barely flow out of the jug.
Here in OK during the summer even 20/50 can really thin out on a 113 degree day.
You should see what sub-freezing temps do to the straight 60 weight oil my old Harleys use. 
A V6 Ford Ranger came in on a hook with a rod knocking and no oil pressure on the gauge. The temperature was below 20 and the oil in the pan was as thick as molasses. The driver said it had 20-50 Castrol and a quart of Lucas oil stabilizer which is similar to STP. When disassembled the oil pump drive was wound up like a cork screw, pulled out of the pump. The oil viscosity was too thick for the pump to turn… Similarly, a Mazda 626 arrived on a hook as a no start. After sitting in the shop several hours, when the starter was engaged the engine started with no problems indicated. After the usual inspections it was road tested. parked and the owner called. The next morning, with the temperature below 20, the customer attempted to leave but the car wouldn’t start. Somewhat embarassed I pushed it back in side, but when able to check it, it immedeately started and ran perfectly. I left it outside that night and immediately pushed it in and tested it the next a.m. It would not start and was found to have nearly 0 compression and the oil was very thick. The owner was using 10-40 oil and adding Lucas and the heavy viscosity was pumping the lifters up, not allowing the valves to seat. After sitting in the warm shop the oil was thinning enough to start.
I will add, with several years owning and maintaining commercial ligt duty trucks and many more years maintaining several large fleets, it appears that 10-40 oil of any major brand, if changed regularly, can keep an engine running 200,000, 300,000, and even 400,000 miles. This is in Northern Mississippi where it rarely gets below 20.
20W50 isn’t heavy. straight 30 is a lot thicker. He’s well within sensible limits, even with the STP. He wouldn’t need the STP with the straight 30. It’s a 1978 car, the engine has a long stroke with a heavy crankshaft and it hardly matters what oil he uses except to never use 5W30 or thinner oil.
20/50 oil is called for in many older european cars from new (in my eyes 30 years is not old). The old Morris/Austin A and B series engines calls for 20/50 oil all year round down to around 10 degrees F. My old faithfull (Morris Minor) drops about 0.5 kilo if I put - say 15/40 oil in it, even from new and I haven’t had any wear on an engine of this type that I can related to too thick oil, not even after 200k - 250k miles.
One issue with older cars without roller lifters is that the oil manufactures have reduced the amount of zinc in the motor oils as they found out that zinc can lead to catalytic converter failure before the emissions warranty is up on newer vehicles. Zinc provides a protective barrier especially between the lifters and the cam preventing metal to metal contact.
There have been some issues with cam lobe wear on older vehicles using the current motor oils.
Adding a can of STP is good for these older cars as it contains zinc which will raise the zinc level in the oil.
I live in hot Florida ( going to hit 95 this weekend) and this is what I use in my cars.
59 Thunderbird, straight 30w with a half a bottle of STP ( owners manual even says you can run straight
40w in the summer 90 degress plus ) 74 Nova, 20w-50 year round with half a bottle of STP. 20w-50 is also stated in the owners manual for constant temps above 60 degrees.
1989 Mustang GT with 105k using 20W-50 in the hot summer and 10w-30 in the winter time when temps are in the 70’s. I have not had any engine problems what so ever. Owned the Mustang & Nova since new, and the 59 bird for 2 years. These cars have been restored to showroom condition.
On the T-bird the owners manual does not even show multi-vicosity oils. It states different straight weights for different temperature ranges. One thing I would not do is, it states to use a 10w straight oil if temps are 32 degrees and below… In this situation I would use a 5w-30 or 10w-30 weight. No way would I run an engine on a straight 10w oil.
On newer vehicles with very tight tolerances I would use what the manufacturer recommends. Now of you have a high mileage newer vehicle and you’re burning some oil it won’t hurt to go up one grade in viscosity.
I have to respectfully disagree with the motor oil zinc business.
I spoke with the neighbor this morning . . . and asked him about the heavy oil mixture and stuff. He told me that he’s been using that mixture since new . . . it’s a 1978 Dodge slant six . . . and he’s checked the valve lash on it every year or so, says it’s in spec. He said he just doesn’t like the “tapping” noise in the Summer . . . and goes to this heavy mixture May 1st, then changes to 10W40 on October 1st for the Winter. He also said that it burns no oil, and that the odometer stopped at 166,000 over ten years ago. Gotta love those old pickups. Rocketman