I can't argue with the calculator. This engine is VERY tight from the factory anyway (Lexus 1UZ-FE), tighter than the similar engine family 8 years later. So this one I can see some improvement. I personally won't be 100% convinced for around 10k miles, which happens fast enough. Either way I get 425 miles to a tank now much more commonly than I did before, if I got to 400 before I was ecstatic. I'll dig up my mileage book and post some numbers of what it is showing right now.
Since I got the car I have run Mobil 1 0W30 AFE, then Quaker State 5W30 syn, and then the last oil change I went with plain jane Mobil Super 5W20. Figured I would experiment on it and see. I am at 170k miles now. I got the car with 138k and change oil right around 10k. It was changed just before I got the car.
I have done a TON of statistical analysis of mpg. I commute 40 miles each way a day, before that 75 miles each way and before that 60 miles each way. So I have taken the opportunity to learn what affects mpg and what doesn't. In my experience, oil weight hasn't had much effect, from 0W30 to 15W40, even in dead of Wisconsin winter. But my drive is unique, start up cold and within 3 miles I am at 55 mph, and 6 miles, 75 mph. With the car I had then (the 94 Fleetwood) it had the oil cooler, so oil was heated by coolant, but it was limited to the thermostat's 180F (stock in the LT1) temps. So it NEVER had a reason to get hot. I did air filters, speeds, electric fan vs mech fan (with a properly operating clutch), 10% ethanol fuel vs 0%, acetone in the gas vs not, oil weight (0W30, 5W30, 10W30, 15W40, 5W40, even straight 30) and other things. Few had effect on mpg like ambient temp and driving style. Engine oil was one that had effects in the less than 0.2 mpg range.
I used Minitab 15 to do my mpg analysis. I looked at it much much closer than anyone else. So seeing a bump in mpg like this is puzzling, it ~shouldn't~ in my experience. I drive sane most of the time, driving gentle sees 27 mpg. Driving normal (65 mph, passing people as need on back roads), 75% higher speeds, some stop and go. With 5W30 and 0W30 I never got to 27 in this type of driving, never ever. I have seen 27.1 to 27.4 in this driving. Same drive with the 0W30 and 5W30 I typically got 22 to 24, and 24 was a rarity. Driving more agressively on the 5W20 I am in the 25.3 range.
I'll go punch in the numbers in Excel and see what I find out for trends. We had a hot summer this year, AC on as usual, yes, AC wasn't needed as much when I put in the 5W20, but it has NEVER got this sort of mpg with AC off no matter what I did.
Oh, spark plugs are std NGK's, and have 30k miles on them. I aligned it this summer (I do my own) and that could be a contributing factor, but I had 5-7k miles on it with the alignment done, again, nothing stood out after the alignment improving mpg.
Oh, air filter also has 30k on it, I blow it out now and then, but it isn't anything special (Wix std replacement)
First oil change was 0W30 AFE with a Toyota std filter
Second oil change was 5W30 with a 51348 Wix filter
3rd oil change was the 5W20 with a Wix 51516. I won't use the 51516 again, it is larger yes, but it is a 21 micron filter and the 51348 is a 15.
I have seen 30.x mpg highway with the 5W30 oil once, and high 29's too.