Dear Ole Mechanic;
What oil should I use in my "94 Chevy pickup?
A Ranchin’ Mom
Hopefully this is a slippery subject without a slippery answer. Of course, I'm going to send you to your truck’s owners manual for general guidelines, but I also understand that it may be long gone in a ten-year-old pickup. Plus, there are so many different types and brands of oil available that it's easy to get confused. Here is my best guess. Your owners manual probably called for a 5W-30 viscosity oil with an API rating of either SG or CD. Now I'll explain the gibberish in that last sentence. The viscosity refers to how thick the oil is at different temperatures or, in other words, how easy does it pour or flow when it's hot or cold. The number ahead of the ‘W’ is the winter or cold flow viscosity. At room temperature, a 5W oil will flow almost like water. But when the temperature gets down around freezing, it get thicker but will still flow OK. The 30 in 5W 30 refers to the flowing ability when the oil is hot--about 212 degrees. No, the oil does not get thicker as it gets hot; it is formulated so that it just does not thin out as much. A 10W-30 oil is thicker when it is cold than a 5W-30 oil, but when hot they should flow the same. As the numbers go up, the oil gets thicker. Now for all those letters. API stands for the American Petroleum Institute, which independently tests oils to see how well they hold up. The S in SG indicates that the oil has been tested for use in Spark-Ignition or gasoline engines. The C in CD indicates that it has been tested for use in Compression-Ignition or diesel engines. Since you did not tell me which engine your pickup has, I am covering both gas and diesel engines. The letter following the S or C indicates the testing level that the API has done and rather than getting into the details of dilution resistance and film strength, it is easier to say that the further along or up in the alphabet the letter is, the better the oil is. An SA rated oil would be fine for the Model T Ford that runs around Harper, but don't try to use an SA oil in your 1994 truck. Actually, the letter following the S or C can and does change, regularly as car and truck makers put increasing demands on the engine oil. Many times an oil can (oops, I am dating myself) an oil container will have both S and C ratings which means that it has been tested and is OK for either gas or diesel engines. Just make sure that the letter following the S or C is high enough for your pickup. That would be an SG or CD in your case. If the letter is further along in the alphabet then that, the oil is OK, as it means that the oil has passed all of the lower tests, also. One note for the Ford Powerstroke readers out there. The oil for the Powerstroke engine(and some International and Caterpillar diesel engines, as well) needs a CF-4 or better rating. The second letter is not that critical as long as it is higher, but the -4 is critical as the Powerstroke will run lousy after 2,000 to 3,000 miles if it is not used.
A note of caution here. If an oil only has an API rating of C something and no S something rating, DO NOT use it in a gasoline engine. It may damage internal engine parts like the camshaft. If the oil only has a S something rating and not a C something rating, DO NOT use it in a diesel engine, as it may damage internal engine parts like the crankshaft and bearings. However, if you get stuck out in the pasture and are way low on oil, then any oil is better than no oil. Just change it as soon as possible.
You may have noticed that I have not said much about the viscosity or 5W-30 part. Now I will. Single viscosity rating oils like the old “20 weight” or “30 weight” and non-detergent oils are still available but do not have a high enough API rating for your 94 Chevy, so don't use them. Since there are viscosity ratings from 0W-30 to 20W-50, the choice is difficult. but I will give you my thinking (since you asked for it). The 5W-30 recommended for your truck would be fine IF you drive at moderate speeds (55 mph), do not use the air conditioner, do not pull a trailer or carry a load. OK, now for Texas reality. Due to the temperatures that we encounter during our summer, you should probably use one viscosity thicker or a 10W-40. If you are going to pull a four-horse trailer with tack at freeway speeds to Houston in the summer, then 20W-50 might not be a bad idea. Heat causes oil to break down to a thinner viscosity, so it should be changed regularly. High mileage engines with a lot of wear may require a thicker or higher viscosity oil just to keep oil pressure up.
Now a little about synthetic oils. I would recommend that you not try a synthetic oil because of the age of your truck, unless you are already using it. Synthetic oils are very good but very expensive, and a change from regular mineral oil to a synthetic oil in a higher mileage engine (50,000+ miles) may lead to increased oil consumption. The best time to switch to a synthetic oil is after the second or third oil change in a new car or truck. Using a synthetic oil will usually result in better fuel mileage and a cooler running, better performing engine. The oil changes can be extended, too, but the trade off is the higher initial cost of the oil.
What brand of oil should you use? I am not an oil salesperson, but I would use and recommend any name brand. I would suggest that you not change brands of oil unless you are having a problem with your current brand. Also, do not go to a thinner viscosity oil unless you are planning a trip way up north in the winter.
If you are asking what oil to use because you have to dump in two or three quarts with every fuel fill, and your truck marks its spot with a puddle of oil, then there is 60 weight oil available.
Herr Professor Nuzanbolts