im noob when it comes to oil. i been always using 10w 40. alot of people use 5w-30. can someone explain wats the difference? is one thicker than the other?

The higher the weight, the thicker the viscosity. Lower the weight, the thinner the viscosity. The first weight number indicates how viscous the oil is when in its cold state, and the second number indicates the viscosity when the oil is hot.

So that oil you're using (10w-40) is a lil bit thicker than 5w-30, in cold and hot state. Thicker oils can bog the engine down a lil bit, IMO just run what is suggested next time. Running 10w-40 won't break anything, it's just a bit more thicker than what's recommended.

Especially since your in Cali, a thicker oil shouldn't make much of a diff. You might notice some slight (very slight) MPG increases with 5W vs. the 10W

Actually, it's a little more complicated than that... in a 5W-30, the 5 is based on how thick or thin it is, the 30 is how much "stickiness" it has in comparison to straight weight oils... in essence, a 5W30 flows like a 5 weight, but has the protection of a 30 weight...

Well not that it matters but using the wrong oil can cause you to lose 2 hp lol.

That's another way to think about it. I think your explanation is better than mines.

I was using a synthetic 5W-40 in my SC400 and it seemed to be too thin because the oil sump pan showed a bit of seepage and I could occasionally smell the oil through the vents. I was thinking of switching to Redline 10W-40 here in WA... What do you all think?

oil leaks aren't typically caused by using the wrong weight of oil... they just happen over time period...

Yeah I know they do, but can't an oil eventually seep out if it's TOO thin? My buddy said switching to a heavier weight may help a bit, but wasn't guaranteeing anything. I figured it would be worth a shot, but just don't want to put TOO think of an oil in, ya know? Think 10W-40 is too much for a cooler climate?

i'm using 10-40 semi-syn castrol in my 1UZ

as with anything, the smaller the molecule size, the smaller the hole is needed for something to leak... switching to a heavier weight oil may slow the oil leakage, but it probably wont stop it... engine oil seals are meant to withstand whatever weight oil the engine was designed around... Lexus are all 5w-30 or 5w-20 pretty much...

The best weight oil to use is the lowest weight oil that is within OEM recommendation. Let me explain; up to 80% of engine wear is from cold starts. This does not only pertain to cold weather. Cold start means the first start of the day after the engine has completely cooled/drained down. This wear accrues in the time the oil takes to get to the vital parts of the engine (bearings, cams, lifters, etc). The quicker the oil gets to those vital areas the less wear. That being said, the lower the weight the better/faster the oil will flow during start-up. The IS, along with most modern engines are using 5w-20. The “w” represents winter and is the important number to look at when determining cold start flow. The lower the number the better cold flow. I personally use 0w-20, which is within OEM specs. There is no need to use 30 weight oil in the IS engine, but it will not hurt the engine. If you feel better using 30 weight oil go with the 0w-30. Any of the top brand synthetics are overkill especially if you are changing your oil in less then 10,000 miles. If you are using quality synthetic oil and a quality oil filter you can safely use a 10K mile OCI, anything less is a waste time and money. For the IS, 10K miles or one year is the rule of thumb.

Koz

To answer your question; Grade/weight of oil – Viscosity (flowability) is how to measure the shear strength of a thin layer of oil. A bearing works by using a thin layer of oil not by the oil pressure generated by the pump. If you use too high a weight oil the layer will not form fully or may not get into the bearing at all. If you use too low a weight the layer will not give the bearing the proper protection. Bottom line, make sure you stay within the manufactures recommended ranges.

Koz

one thing to consider is getting your oil analyzed by a lab like blackstone. it's only like $22 and they tell you how your engine is wearing, how much longer that oil coudl have protected your engien and recommend an approximate oil change interval based on the results. many people change their oil way too often in these days of superior oil technology and better engineered and built engines.