Hey Guyz,
Right now I have Castrol 20-50wt running.
What does Cadillac call for viscosity wise for the 500CU, I have a "75" 500 Carb'd in my Coupe.

I see people talking about Royal Purple and Amsoil, which is the better synthetic and what weight oil for these guys ?

All advice is appreciated, thanks........

Oo, that's some thick oil. Bad for start-up wear, and bad for fuel mileage. I don't have the factory spec for the 500, but I'll bet it is 10W30.

Amsoil and Royal Purple are "premium" synthetics. They are both very good. I use Castol Synthetic (German) in my LT1 powered SS. I am currently using 5w40, but I am going back to 5w30 next change to see if I can pick up a little mileage. I went with the 40 because I hammer that car (standard trans) and it regularly spins over 5500 RPM and I wanted the protection that a slightly higher hot viscosity offers.

If you are just using it in warm weather, go with a 10w30. If you drive it in cold at all, consider a 5w30.

I wouldn't be surprised if GC was great in a 500, too...

Oo, that's some thick oil. Bad for start-up wear, and bad for fuel mileage. I don't have the factory spec for the 500, but I'll bet it is 10W30.

Amsoil and Royal Purple are "premium" synthetics. They are both very good. I use Castol Synthetic (German) in my LT1 powered SS. I am currently using 5w40, but I am going back to 5w30 next change to see if I can pick up a little mileage. I went with the 40 because I hammer that car (standard trans) and it regularly spins over 5500 RPM and I wanted the protection that a slightly higher hot viscosity offers.

If you are just using it in warm weather, go with a 10w30. If you drive it in cold at all, consider a 5w30.

Thanks for the tips.
I am not worried about gas mileage as I don't get any anyway, ha-ha, but I do worry about engine wear, you say 20-50 is bad for start up ?
Where I live the summers are 95-110F and the winters are about 25-50F.

Also if I were to go with the Royal Purple, what weight would you recommend ?

Thanks.......

5w30, 10w30, 5w40, 10w40... pick one and you'll be fine. Personally, with your climate, I would choose 5w40.

Does anyone have a owners manual for a 75 Cadillac 500CI and know exactly what GM advised what oil weight to use ?

i put castrol synten 20w50 in mine. runs great. i'm thinking about downgrading to a 10w40 when i do my engine build.

and yeah i have a 1975 carb'd 500 as well

I appreciate all the tips, but does anyone know the GM oil viscosity/wight for the 1075 year 500 engine ?

I am very curious as to what original weight was....

The original weight was 10W30.

I have used 15W40 HD oil in all my cars for 25 years. Never had an issue with any engine I've had.

Mike

The original weight was 10W30.

I have used 15W40 HD oil in all my cars for 25 years. Never had an issue with any engine I've had.

Mike

Thanks for the original oil info and all the other info Guyz !
I really appreciate it.....

I am a fan of 15W40 in my older cars..... Rotella T synthetic 5W40 or Rotella T 15W40 petroleum. In my newer cars, Amsoil or Mobil 1 5W30.

I'm using 20w-50 in mine its a '76 fleetwood with the carb, according to the handbook its got that chart with all the overalpping oil types and temperature ranges, i'm in a climate which has frosts, rare snowfalls and hot summers.

Does anyone have a owners manual for a 75 Cadillac 500CI and know exactly what GM advised what oil weight to use ?

The manual for my '76 I believe says 10W30. Personally I usually use 4 quarts of 10W30 and one quart of Lucas Oil Stabalizer........

That stuff is no good...

I still haven't figured out how oil CLINGING to the parts is better than it getting back into the pan and being circulated properly. It makes the parts heavy, out of balance and the oil likely hotter.

For differentials and gear boxes, sure, seems like it makes sense, for engines, no, doesn't make much sense. But my kids sure like the little gear thingy at the parts store!

20W50 modern oil is probably fine, the rumors I heard was the 20W50 and 10W40 had ring sticking problems with the added additives in it, so 10W30 was the preferred oil and kept warranty issues down. Gotta admit, I haven't seen a sticking ring in a long long time....

FSM says;

"In areas where temperature seldom drops below zero, most 10W or 10W-40 are satisfactory for easy starting of the engine. When the temperature is frequently near or below zero, a 5W-20 or 5W-30 oil is recommended. (Note: 5W-20 oils are not recommended for sustained high speed driving. Non-detergent and low quality oils are specifically NOT recommended for any type of service.)"

I tried to copy and paste that from the FSM, but it wouldn't let me so I typed it verbatim (including the 10W mistake). Hope this helps.

Interesting, 5W20 back then was even recommended for certain conditions!

I have wondered about using it to help mpg, but sorta scary to use water thin oil....

Yeah according to the chart at the back of the section where i found the inforamtion i listed earlier, the 5W-XX is only recommended from temperatures between -20 and 20. It makes a statement about the car being operated mostly in Canada.

I have heard of some racers using 5W20 in a regular engine that calls for 10W30, but it still scares me some. I would have to see more data on it for a modern car. To be safe, probably wouldn't...

In cold cold climates, yes, I would probably run. IF keeps cold all the time where you just need the flow over the weight of it.

I'll agree with that. However, i have seen some new cars that recommend 5W-XX as the norm. I have had two myself. Now I didn't use it, I use 10W-30 most of the time. I don't think the small amount of mileage increase is worth the possible extra wear from the thinner oil. Besides like someone said above, on a '75 lac gas mileage is an after thought. I checked mine the other day, i get about 12.5 MPG. Now mind you it needs some tuning but I don't expect more than 15 if that.

FSM says;

"In areas where temperature seldom drops below zero, most 10W or 10W-40 are satisfactory for easy starting of the engine. When the temperature is frequently near or below zero, a 5W-20 or 5W-30 oil is recommended. (Note: 5W-20 oils are not recommended for sustained high speed driving. Non-detergent and low quality oils are specifically NOT recommended for any type of service.)"

I tried to copy and paste that from the FSM, but it wouldn't let me so I typed it verbatim (including the 10W mistake). Hope this helps.


Hmmm, sounds familiar:thumbsup:

The higher viscosity oils have cling properties and will adhere to metal surfaces. Will stay in areas of close clearance, like between bearings and crankpins when the engine is idle. Your lower viscosity oils will drain off and over time leave your surfaces with little lubrication for starting. Your higher viscosity oils have a higher molecular density and will stand up better for high engine speed operations and higher operating temperatures.
Unless you've added gauges to you Cadillac, the only monitering of your engines are the idiot lights on the dash. If you find a drop of 20#s in oil pressure from the time you turn the key to the time the engine reaches operating pressure, a rule of thumb suggests that your bearing clearances are reaching the point where you need to address replacement and possible repair. In this case I have advised people to go to the higher viscosity oils and to keep engine speed lower, so as to keep from doing more damage until able to do the needed repair.
I know some would debate this with the argument that the higher viscosity oil would cause the bearings to spin. If you've reached the point where the clearances are that extreme, you have a choice of using the lower viscosity oils and hammer your moving parts to death or using the higher viscosity oil to quench and cushion the impact between the parts.
In an engine with unquestionable condition, I like to use the 15W/40 in weather where temperatures are averaging 70 and above, and 10W/30 in temperatures averaging below 50 degrees. There are also arguments against changing oil through the Winter months, but up here in the Northeast temperatures can go as low as -10F in Winter and I feel a lighter oil is beneficial for easier starts at these low temperatures.
I have used some synthetic blended oils and haven't made a decision on their use. If you go this route, look for oil leaks you didn't have before. No one has come up with a good way to check condition of oil in use. You could check viscosity with a refractor, but I don't have a usable scale. I have used PH paper to check acidity, but it was out of the range of the paper. Synthetic oils last longer, but if either of these two conditions reach critical
range, it should be changed.
Perhaps someone here could arrive at a solution oil could be changed on either one of these two criteria.

Amsoil now sells 10W40 in a "high zinc" formula. Sounds like a winner for old cars to me!

Amsoil now sells 10W40 in a "high zinc" formula. Sounds like a winner for old cars to me!

I may have to look into that one.
Anyone running this in an older 500 engine ?
Hence I only have 66,000 on mine, it's still old, lol.......