Continuing this off-topic discussion of TooSlick's formula: plugging some more numbers using most recent Butler VOA findings, and a new one for the Amsoil ACD Heavy-duty SAE30/10W-30, we get:
Mobil 1, 0W-40----13.9/3.6 = approx. 3.86
Mobil 1, 5W-40----14.5/4.1 = approx. 3.54
Mobil 1, 10W-30---11.0/3.2 = approx. 3.43
Amsoil SAE 30-----10.9/3.4 = approx. 3.21
Amsoil 3K 5W-30---12.2/3.6 = approx. 3.38
Notes:
(1)Amsoil HDD is shown at 11.5 on the Amsoil web page, but reported by Butler at 12.2. At 11.5, the ratio would be 3.19. (If the HT/HS is 3.6...on the web page, it is reported as >3.5, which could mean 3.51 and above).
(2) Amsoil promptly emailed me the 3.4 HT/HS for the ACD. This number is NOT reported on their web page.
I conclude from this, that this nifty formula is a good relative gauge of shear stability, and heavily dependent on the accuracy of the reported numbers. Also, the table above - reflecting a few tested numbers (more accurate?) - tends to squeeze the ratios closer together. Of course, the greater the vicosity range, the generally less the indicated shear stability, as expected (the Amsoil 5W-30 ratio - using 3.6! - is an exception above v. the Mobil 1 10W-30...if 3.51 to 3.59 is used, the ratios again only reflect the viscosity spread).
Mobil 1, 0W-40----13.9/3.6 = approx. 3.86
Mobil 1, 5W-40----14.5/4.1 = approx. 3.54
Mobil 1, 10W-30---11.0/3.2 = approx. 3.43
Amsoil SAE 30-----10.9/3.4 = approx. 3.21
Amsoil 3K 5W-30---12.2/3.6 = approx. 3.38
Notes:
(1)Amsoil HDD is shown at 11.5 on the Amsoil web page, but reported by Butler at 12.2. At 11.5, the ratio would be 3.19. (If the HT/HS is 3.6...on the web page, it is reported as >3.5, which could mean 3.51 and above).
(2) Amsoil promptly emailed me the 3.4 HT/HS for the ACD. This number is NOT reported on their web page.
I conclude from this, that this nifty formula is a good relative gauge of shear stability, and heavily dependent on the accuracy of the reported numbers. Also, the table above - reflecting a few tested numbers (more accurate?) - tends to squeeze the ratios closer together. Of course, the greater the vicosity range, the generally less the indicated shear stability, as expected (the Amsoil 5W-30 ratio - using 3.6! - is an exception above v. the Mobil 1 10W-30...if 3.51 to 3.59 is used, the ratios again only reflect the viscosity spread).