How do I convert a specific gravity of .985 @ 25C to what the specific gravity would be @ 15C?

Generally you can not. You need a reference like the chemistry and physics handbook to look this p for the specific material of interest.

Try some heat transfer data sheets which plot SG versus temperature for typical mineral oils.
Also coeff of vol expansion for HVI iso 22 oil is typically 0.00075 per deg C(from same data sheet)
If my simple logic is right then SG at 15C is approx 0.992 (0.984/(1-.00075x10)
0.00075 not necessarily the exact coeff as SG of this oil differs.
All this from memory so advise check up.

Try out this one.....

Density at 29.5°c = (Actual temp - 29.5°c)*0.00065 + Actual density

density at 15°c = Density at 29.5°c + 0.0094

Regards,

CW:

You can visit the site of jmw,http://www.viscoanalyzer.com/page8.html, where you can download the spreadsheet for Density calculation and Viscosity calculation.

Good luck !