STP is an interesting product, and few people understand it, so let’s get rid of some of the nonsense. First one person said to thicken oil simply use 2 cans of 30 weight oil, this isn’t a good thing to do for modern cars where the owner’s manual recommends 0-20 or 5-30, straight 30 could cause some fatal damage to engine. Now try this, take a can of 30 weight oil, or even your favorite oil regardless of weight and get some on your naked, yes that’s right I said naked, index finger and thumb and rub them together, feel how slippery it is, then wipe the oil off with a rag and try doing the same thing with STP, you will discover something interesting…it’s slippery then your engine oil even though it feels a bit thicker. I would not use any oil additive in a modern car due to the tight tolerances in engines today, but if you have an older style engine from the 1980’s and older and you have issues with it burning oil than STP might help to reduce the oil burning.
I think too that with modern engines that use 5-20 or 5-30 engine oil as the miles tick up eventually you will want to shift to a high mileage oil at probably around 100,000 miles; and shift to the next higher weight oil, so from 5-20 to 5-30 for example, when it starts using more oil than normal. 5-30 is still thin enough to be plenty safe for winter use. Typically you should never use any oil thinner or thicker than the car manufacture states to use, but when the car starts to use more oil between oil changes that is due to more space, or slop from wear going on inside the engine, at that point it has enough wear that the tolerances aren’t as critical as they once were and the next step up in weight won’t hurt the engine and will protect it longer. Once you go to the next thicker oil you’ll need to watch the oil usage closely to get a baseline, once you have that and later, say 100,000 miles later and it’s using more oil than what that baseline gave you then you can go to the next weight up like 10-30, by the time you get to that 10-30 weight oil you should be pushing around 300,000 miles depending on how the engine is using oil of course. Most new cars today do use oil naturally as brand new engines so you have to know you vehicles oil usage really well because you don’t want to jump up to the next weight shortly after you get the new car, that thicker oil could ruin your engine and void your warranty on top of it!
I had a vehicle once that never used any oil between changes for 240,000 miles! So all I did was switch to a high mileage oil at the 100,000 mark. I sold the vehicle so I don’t know how long the engine either lasted or if it started to burn oil at some point.