oil weight, cold vs hot
 

i just changed the oil in the svx recently, and used 5w30 instead of 10w30, because my store was out of 10w30.

my question is, when i was doing some of my studying for a test at work, it stated that in multi weight oils, the first number is only the weight at cold temp, saying it wont go heavyer than that at winter temps. so does this mean that during summer time, 5w30 and 10w30 are the same?

i may have read that paragraph wrong, which is why im asking.

Re: oil weight, cold vs hot
 

5W30 will still be a little thinner at start-up but will be the same viscosity at operating temperature. I use 5W30 in all my vehicles year round. It never gets above 90°F here for more than a few days.

Re: oil weight, cold vs hot
 

I us 10W40 all year round.. That bad?

Re: oil weight, cold vs hot
 

That's all I've ever used in anything.

dcb

Re: oil weight, cold vs hot
 

The oil has the same weight cold and hot, density and expansion is another matter. :rolleyes::p

Re: oil weight, cold vs hot
 

the 30 is the oils base weight...... how the thick the oil stays when its hot. The 10 and 5 are the oils lowest viscosity with the added detergents. Basically you are using 30 weight oil that has been thinned out to flow like a 5 weight when its cold, thru the use of detergent additives.

Personally id never go back to a 5W oil on anything over 100K, it will exacerbate any leaks, and it also does not cushion engine parts as much, and your oil pressure will be lower when your car is hot.

Re: oil weight, cold vs hot
 

For choice of oil weight, a lot depends on where you live, and how you drive is also a lesser factor.

As others have noted the numbers refer to the viscosity (thickness) of the oil under different conditions. For a multi-grade oil like 5W30 or 20W50, the first number relates to (but isn’t a measure of) the viscosity when the oil is cold. The lower the number, the less viscous (thinner) the oil, and conversely, the higher the number, the more viscous (thicker) the oil. BTW, the “W” means “winter” not weight, and describes an oil which has additives which help deal with condensation.

Engines need oil that is viscous and wetting enough to cover all moving surfaces to minimize wear, friction and heat, but not too viscous that it can’t be pumped through the engine lubricating channels.

So if you’re lucky enough to live in FL or So Cal, you can safely use a 10W30 oil year round and are probably better with a 10W40, or even a 10W50 or 20W50 if you drive hard (high RPM/hot engine, etc). But if you live in the northern tier states or Canada, you need to use a 5W30 or 5W40 in the winter. The lower first number for these oils (“5”) means they behave like thinner oils when cold, and so can be pumped and coat the metal parts (think of a cold thick oil being like molasses), but the second number (“30” or “40”) means that the oil will still behave like more viscous (thicker) oil at full engine operating temperature (ie. it won’t end up very thin like water so it still has some coating properties even at higher temperatures).

Worn engines can achieve lower oil consumption by using a heavier weight oil (ie. going from a 5W30 to a 20W50 will help), but if you live in a cold climate, your engine may not even turn over properly if you use a heavy oil, and in the end you’re delaying the inevitable.

For racing, sometimes “straight weight” oils like 50 weight or 50W50 are used, particularly in older engines. This is because they are not started nearly as often as a daily driver when cold which is when you need the thinner oil to prevent wear, and because the viscous heavy weight oils like a 50 weight will stand up to some extremes of heat and high RPM.

Gear oils are usually much heavier weights (eg. straight 90 weight or 100/145) and should never be used in an engine!

Very modern engines with finer tolerances and better heat dissipation are often designated to run on 5W30 or even 5W20 oils year-round. Lower viscosity oils like these do give slightly better gas mileage, but aren’t appropriate for older engines, especially ones with wear and/or leaks.

A word on SYNTHETIC oils -- these have a different chemical base and were originally developed for turbine lubrication (think 25,000 RPM and up!!), for example in jet engines. They are very suitable for car engines, but be aware of some CAUTIONS:
(1) For the same “weight designation”, synthetic oils behave as if they are less viscous (less thick), and so any wet engine will likely leak like a sieve if you use synthetic oil.
(2) Synthetic oils are NOT COMPATIBLE with the oil seal materials in many older engines, and can actually damage some types of oil seals. Most engines manufactured after 1990 are OK for synthetic oils if the engine is still very tight and dry or fully rebuilt. BTW, always run a new or rebuilt engine in using conventional mineral oil for the first 5-10,000 miles; it allows the metal parts to “bed” properly. Then change to synthetic if you want to.
(3) Synthetic oil manufacturers claim that you don’t need to change the oil as often as for conventional mineral oils. This is true when oil breakdown is due to mechanical shear caused by the engine parts moving, BUT if you drive in dusty conditions or very short stop-start runs where dirt and condensation can build up in the oil, or you suspect any sludge build up, synthetic oil will not protect your engine any better than a good mineral oil. Change the oil!

BOTTOM LINE -- USE a TOP QUALITY oil, either conventional or synthetic, of the right weight designation, and change the oil AND filter very regularly. I’ve had engines with over 300,000 miles on them which don’t use or burn a drop of oil between changes.