Originally Posted By: 06VtecV6
Guys, I wouldn't have believed it unless I just saw it unfold and and bloom this past few weeks. I'm on (and been using 5w weight to the exclusion of 10w FOREVER) a 5w-30 fill and getting no better then about 17.xx mpg. Ever since I topped off with 10-30 PYB, that mileage has creeped up to about 19.1 mpg! This is with only using 1 quart of PYB 10-30. Original fill was 5w-30 Val full syn. I am sold on this 10-30 weight now for S to SE texas and I hope to turn the corner with my oil performance(s) knowing that 10-30 is the right VISCOSITY for S (SE) texas and not the 5w-xx [censored] the car manufacturers have been throwing at us for decade(s) now! Now granted the PB may have also had the right additive pack that this Lex/Toy responds optimally and ideally but never thought 10w wold increase my FE. I was under the impression that I had to go lower viscosity on the cold side to achieve better FE but now I realize the folly of my way was not understanding that lower cold viscosity or "thinner" oil doesn't "automatically" increase FE. This may have been self-explanatory to all but I was not aware that if you get no cold temps AT ALL, you should stay away from 5ws and 0ws completely, just was never clearly explained. Was I so clearly deluded and easily misled by the oil cap and the car's manual and not on my research here at BITOG, why yes. I should have known Rule #1 and that is climate and duty cycle dictate the oil viscosity weight and not what the manuf says as a blanket rule for ALL climates and this should be considered first in the correct logical order of choosing oil. Viscosity per your climate and the heck with what the manuf calls for whether its for CAFE or simply because most cars toil in moderate to mild temps with cold being the extreme not HEAT.
First but not least, your measurement of fuel economy have about 15% experimental error. Therefore, any increase or decrease in fuel economy that you see is not meaningful. It's impossible to do fuel-economy comparisons unless you install a precision fuel meter and do very controlled experiments in exactly the same driving conditions (speed, acceleration, grade, altitude, temperature, idle time, engine temperature, etc.). Unless someone reports experimental error in an experiment along with the mean value measured, the results don't have much meaning.
Second, if you believe that 5W-30 is too thin for your engine and causing excessive metal-to-metal contact, which is decreasing the fuel economy, why not try 10W-40, 0W-40, or 5W-40? There is little difference between 5W-30 and 10W-30. You can also run a synthetic 5W-30, which doesn't shear and will stay as thick as a conventional 10W-30, and you get better cleaning, fuel economy, longer OCI, less wear, etc. with synthetic.
https://en.wikipedia.org/wiki/Observational_error