I know this has to be a pretty common question, but I would like a little direction as to the proper oil to use on my 1976 Firebird T/A (weight / synthetic vs. conventional, etc). Every forum I’ve been on gives a different answer (almost every line of the thread) and most explanations seem to be “…I’ve been doing this way for year and never had a problem…” or when they do offer more specific information, the next poster says that it’s wrong. Please help!!
The engine is a '73 Pontiac 400 with a mild build (bored .40 over, street cam, aluminum RPM performer intake / carb, headers etc.) The car is garaged all the time and driven 1-3 times a week until around October, then it’s stored until the snow and sand is gone in the spring.
I would probably use 10W-30 or 10W-40 conventional motor oil, but that is just one man’s opinion.
What really counts is the opinion of the folks who designed and manufactured your car.
So, I would suggest using the type of oil that is specified in your Owner’s Manual.
However, in view of the age of the engine, I think that it would be wise to use a ZDDP additive, in addition to your choice of motor oil. This is the stuff that I am talking about:
Pennzoil does not have ZDDP in it anymore. They called it Z7 but you don’t see that on the bottle anymore.
The owners manual and the top of the oil cap probably call for 10w40, but GM engines after 1975 had trouble with this oil sludging up. They didn’t realize this until about 78 when the customer complaints came rolling in. At that time they recommended 10w30.
The 10w40 of today is not the same 10w40 of the 70’s so as far as I know, you can use it safely today, otherwise go with 10w30, your call. 5w30 would be too thin for this engine.
Once the engine is broken in, you can use either synthetic or conventional. I suspect that you will change your oil about once a year and it won’t have a lot of miles on it. You can go 5000 miles on todays conventional oil in this car with no problems. You can go 7500 on synthetic easily.
If you do less than 5k, there is no benefit to synthetic. If you go between 5k and 7.5k, then synthetic does have the advantage of one oil change per year vs. two.
The decision is really with you, but the real key here is, the only way you can go wrong is to not change your oil on a regular basis. What oil you use is less important than the oil change schedule you use.
I would use 10w30 synthetic. Synthetic does not sludge when sitting for an extended period of time. You’ll get lots of opinions on dino vs synthetic, but I spend the extra bucks and feel safer.
High mileage oil is suppose to have more additives to prevent wear. I recently saw an interview of two Valvoline engineers (Eric the car guy on You tube)who said HM oil is fine in a new engine if you really want to keep the car for a very long time. I found this surprising. I don’t know but I thought I would throw it out for consideration. I do know HM oil is not energy conserving. i.e. your mileage will be slightly lower. But I guess that is not a big concern.
If it was my car I would use 10w30 at a 3k oil change. I have thought about the Castrol high mileage oil, if it is similar in price go for it, I think trying to extend oil change intervals with synthetic is not my cup of tea.
The 5W weight would ensure fast lubrication on cold starts and the 30 weight would provide proper lubrication once the engine reaches operating temperature.
I used 5W30 in my 68 Galaxie 500 with the 390 CID engine for 12 years which was stored each winter. And I drove that car for 12 years without a single engine problem.
These cars had very “loose” engines, and a 10W30 would be good in the summer with a 5W30 in the winter. Normal dino oil recommended; the very slippery synthetics may cause oil consumption.
If you live in a very hot area, the 15W40 diesel “mixed fleet” oil would be a good choice. In Mexico they often put 20W50 in those vehicles.
I’m not as concerned about the weight of the oil as I am about the composition of the chemical additives. The 10W30 of today isn’t the same as the 10W30 of 20 years ago and won’t help the slipper cam in that car as well as an oil that has a higher level of ZDDP, like diesel oil does.
@Tester not meaning to ruffle any feathers, and 5w30 is better in your analysis for sub zero temps. I do not have a problem with 5w30, just saying 10w30 was spec and used it in all my old carsl, but if 5w30 is an added edge I can understand that and do not mind saying if @tester thinks it is ok he is a person I would have most respect for and follow his advice.
My biggest concern is with the lack of ZDDP in modern oils. Classic engines depended on ZDDP to provide protection for the valvetrain. It can be added with an additive or you can find oil with it already in it. Modern engines have been engineered to eliminate the need for ZDDP, but a '73 Pontiac 400, even recently rebuilt, most likely still needs it. With an additive or a substitute, you can use it with modern oils and even use synthetics.
> A 5W oil or a 10W oil is the weight of that oil until the temperature drops to 0 degrees F.
Below that the oil begins to thicken.
…
But once that 5W or 10W oil reaches the bearing journals where heat builds very quickly,
both oils convert to a 30 weight oil.
I’ve never heard it described that way.
Years ago, it was:
A 10w30 has the same flow characteristics and behaves like a 10w oil at 0 degrees centigrade.
A 10w30 has the same flow characteristics and behaves like a 30w oil at 100 degrees centigrade.
The oil rating system has gotten more sophisticated over the years, but I’m not aware the above is no longer valid. If it doesn’t hold anymore, I’d welcome learning how it has changed.
Not that it makes any difference, but is this a Chevy Small Block or an actual Pontiac engine? If you will be driving it in cold weather, then 5W-30 with a zinc additive (Bardhal #1) would be my choice. If not driven in cold weather, (below 20 degrees) then the 15-40 fleet oil works good because it comes with the anti-friction zinc additive. So do motor oils labeled “Racing Oil” but they tend to be available only in single grade…
Docnick, they use 20W-50 on everything in Mexico.a This is a serious cultural defect in Mexico and of no technical merit whatsoever. Even small car shops don’t have 10W-30. I once went looking for some and some young smart aleck snarled at me, “It will destroy your motor!” As if he really knew what he was talking about.