The most important system to the "health" of your engine is the lubication system. Oils have changed a lot in the past few years, and a lot of confusing information is out there. Hopefully this will help.
An oil's viscosity is just how thick it is. Viscosity is measured by an oil's "weight" Way back when all cars used 30 wt (weight) oil. This is a single grade oil: its actual viscosity varies with temperature. A single grade oil gets thinner when hot and thicker when cold. Multigrade oils have labels like "10w-40" or "20w-50". Their thickness changes with temperature also, but they change the opposite way from single grade oils. A 10w-40 motor oil behaves like a 10 wt oil when cold and behaves like a 40 wt oil when hot.
Generally a thicker oil will withstand more abuse, however thick oils can take longer to reach engine parts when the engine is started. In cold climates a thinner oil (lower "wt" number) is often used to compensate for this. Engine startup is when most engine wear occurs: it runs for a second or so without oil pressure every time you start it.
Using a Thicker (Higher viscosity) Oil
Manufacturers are using thinner and thinner oils in cars. (5w-30 wt is recommended for many new cars.) This is primarily for fuel efficiency. In Florida it's usually OK to use a thicker oil than this, but I'm not sure it really helps things. In some newer cars it could cause problems! (see the "Saturn" warning below). I haven't seen any of the new vehicles burn up engines from these thinner oils. I have seen higher mileage engines consume more oil of the lower viscosity types: if your engine uses more than 1 qt every 1000 miles you might use a higher viscosity than recommended by the manufacturer.
Note: Higher Oil Consumption
Partially because of thinner oils some newer cars have higher oil consumption than cars did in the "good old days". The manufacturers say a quart every 2000 miles is NORMAL OIL CONSUMPTION!!!
Oil consumption will increase if cars are driven really fast, or run at high RPM's.
If you're changing oil every 3000 miles this "2000 mile" use of a quart won't be a problem, but if you "stretch out" your change interval to 5000 miles or so you might need to add a quart between changes. CHECK YOUR OIL REGULARLY!!!
Higher Viscosity Oils for Higher Temperatures
Another reason to go with a slightly higher viscosity would be if you anticipate driving in extreme high temperatures. Otherwise, I'd stick with whatever grade the manufacturer recommends. If you decide to go with a higher viscosity oil, don't go crazy! in other words, if a 5w-30 is recommended, increase to a 15w-30, or maybe a 10w-40. an increase to, say, a 20w-50 might cause problems with newer vehicles.
CAUTION TO SATURN OWNERS
I have actually experioenced this one! If you stick 20w-50 oil in Saturns, the engine may never run again!!!! The tolerance is so tight on the hydraulic valve lifters that the heavier oil makes them "pump up" and keep the valves open all the time!
The motor will start up fine, but after it shuts off and cools down, it won't start again!
My customer had knocked a hole in her oil filter and limped home by adding oil she bought at a convenience store. The oil she bought was 20w-50, and although she made it home the car would not start again. I checked the compression, and it was zero on all cylinders. Upon disassembly, the head was OK! Then I read a service bulletin about the lifter issue. a set of new lifters and the Saturn ran fine again.
What about those additives?
There is a bewildering array of oil additives on the market today. They make claims of increased fuel economy and engine wear prevention. Some promise to be an engine overhaul in a can!
Although not all additives are "snake oil", I personally don't recommend them. I say use the oil grade and viscosity (weight) recommended by the vehicle manufacturer. Use the same type of oil all the time if possible. Use a reputable national name brand (any of the major oil companies oils, or independents like Valvoline, Castrol, Kendall, Pennzoil, to name a few.) Stay away from weird or look-alike brands.
Change the oil every 3000-5000 miles (depending on how rough your driving conditions are) or every 6 months. (Cars that sit condense nasty acids in the pan, so do the change on a time basis even if you haven't done the miles)
Additives fall into 3 categories:
1) Additives that thicken the oil
The oldest and best known of these is STP. I know people who swear by it, and I'm not going to argue with them. I'll often use STP as an assembly lube when I rebuild an engine. These additives increase viscosity, and this means more oil will stay on the engine parts longer. This could help in startup (see hydrostatic bearings article).
How it could hurt:
Modern engines have a lot tighter tolerances than older motors. In extremely cold weather, the thickened oil could take too long to get to certain parts of the motor. The 305 Chevy motor was burning up front camshaft bearings up north a few years back. Turns out too thick oil was the cause.
How it could help:
In a high mileage motor, it might just help a bit. Also, in extreme hot weather or racing applications, the thicker stuff might help.
2) Additives that thin the oil
These usually claim to clean the motor, free up lifters, etc. Once again, on a normally functioning motor this shouldn't be needed. Any good motor oil has detergents (cleaners) in their oil. Regular changes will keep your motor nice and clean.
How it could hurt:
Thinning the oil means it won't stay on the engine parts as long when you shut off the engine. (see hydrostatic bearings article). Under extreme loads, the thin oil will break down quicker. Also, the additive may not be compatable with the additives already in the oil. Also, if it did break loose a bunch of sludge, the sludge could clog oil return holes or the oil pickup screen in the bottom of the pan. If this screen clogs, you lose oil pressure, and can destroy your motor.
How it might help:
If you've got a really sludged-up engine, one of these might free your stuck lifters. A lot of these are just kerosene or transmission fluid. I'd recommend a quart of automatic transmission fluid to replace 1 quart of oil on a 5 quart change to free lifters. It will probably work as well as the additives, and is much cheaper. Don't run it forever: just a few hundred miles, then change the oil.
Additives with particles of teflon or zinc
The best known of these is Slick 50. Although great claims have been made for such stuff, I still say stick with just oil. I have yet to see proof that the teflon or zinc actually stick to the engine parts, and it would seem that would be an easy enough thing to have verified by an independent lab. Plus, how do these "micro-particles" keep from gathering in the oil passages and filter?
How it might hurt:
Clogged filters and oil passages with the teflon stuff, plus additive incompatability with the zinc compound stuff.
How it might help:
Once again, it might help a high mileage motor. There are a lot of folks who swear by the stuff, but this could be a "placebo effect!"
How about synthetics?
Synthetics actually are pretty good. You HAVE to use synthetics in Alaska: regular oil turns to jello! Synthetics keep the engine really clean, and they have superior lubricating properties and don't break down as soon as regular oils.
A couple of problems: read the label: many "synthetics" are part synthetic oil, part "carrier oil" whatever that is. The makers don't elaborate, but I assume it's just regular oil. Often the percentage of carrier oil is like the percentage of "inert ingredients" on a can of insecticide! You're getting 5% or so of synthetic.
The only negative (besides price) on the synthetics is oil seals. Although the makers claim to have eliminated this problem, I've found starting to use synthetics in anything but a new or newly rebuilt engine is asking for all kinds of oil leaks.
These leaks won't go away by switching back to regular oil: the seals are trashed. So if your vehicle has more than 50,000 or so miles, or more than a few years old, I'd recommend staying away from the synthetics.
Other Oil Resources
Here's a link that goes into more detail on oil: Motor Oil Myths and Facts
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Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.