If the 30 in 5W-30 is the viscosity of the oil when the engine is warmed up, why would the manufacturer specify 10W-30 if temperatures are above 86F but 5W-30 for temperatures below 86F? I read somewhere that in order to get a lower W weight, they use friction modifiers which make the oil not maintain its capacity to hold foreign material as well, that the bigger the difference, the more modifiers they need.

So I was wondering, it specifies 5W-30 for below 86F and 10W-30 for above 86F for conventional oil, would the use of full synthetic oil be enough to allow usage of 5W-30 for temperatures above 86F?

 

bump

 

Hmm, couldnt tell you really. But I can say that if you are looking for oil that will lubricate, cool, and clean parts really well. I'd suggest 10w-30 Brad Penn Racing Oil. It contains a high concentration of Zinc and something else, which I cant remember off the top of my head right now, but as you may know Zinc is THE number one metal to metal lubricant. Though you may not pass smog with this oil, you can have peace of mind revving to 9k+. I use 10w-30 because I am not very particular as to the amount of contaminants the oil holds, just do your oil change every 2500-3000 miles and sea foam your engine whenever you feel like it. It will keep it nice and tidy internally, as well as performing above and beyond expectations. I might also suggest the use of an OEM S2000 Oil Filter, its a smaller filter, so it increases oil pressure, another step towards never spinning a bearing or grinding cams ever again. Just my .$02. GL.

 

It's not friction modifiers they use, it's viscosity index improvers. Basically, a 5w/30 oil will be made with 5wt base oil so it flows well at cold temperatures. What the viscosity index improvers do is cause the oil to get thicker when the oil warms up so it because a 30 wt oil at 212 degrees F. Multigrade conventional oil doesn't have any less capacity to hold foreign material that other oils, that's dependent on the dispersants and detergents. The problem with multi-grade conventional oil is that these viscosity index improvers are not very durable and they breakdown easy. They are made with material very similar to the rubber on the soles of your shoes. Synthetic oil does not have any of these VII's so it does not break down. If you are in Castro Valley I would just recommend 10w/30 synthetic, whatever brand is on sale.

 

Not that I live in Castro Valley but that place can experience temperature extremes both high and pretty though not freezing low. Also I just read the owner's manual for the same model car that is a few years newer and it suggests 0w-30 or 0w-40 for when driving in extreme conditions, something that isn't even mentioned in the owner's manual for this older car. I guess it makes sense because oils have gotten better and consequently are allowing the use for oils with a lower viscosity.