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HyJet is ExxonMobil's fire-resistant phosphate ester aviation hydraulic fluid. HyJet IV-A plus is an SAE AS1241 Type IV (low density) fluid. HyJet V is a Type V fluid. See the HyJet general properties page for the grades and approvals.
ExxonMobil prints two density facts for each grade. One is the density at 15.6 °C (60 °F): 0.999 g/mL for IV-A plus and 1.000 g/mL for V, both with 8.35 lb/gal. The other is the specific gravity at 25 °C/25 °C: 0.996 and 0.997. The sheets are typical values, not measurements, and they say values may vary within modest ranges.
The data sheets give no density table against temperature. So the line on this page is derived in three steps. First, the specific gravity is multiplied by the density of water at 25 °C (997.048 kg/m³). That gives 993.06 kg/m³ for IV-A plus and 994.06 kg/m³ for V. Second, the maker's mean expansion coefficient for 25–100 °C (0.00086 per °C, both grades) gives the density at 100 °C: ρ100 = ρ25 / (1 + 0.00086 × 75). Third, a straight line is drawn through the two points: ρ = 1013.117 − 0.80228 · T°C kg/m³ for IV-A plus and ρ = 1014.134 − 0.80309 · T°C kg/m³ for V.
As a check, the line gives 1000.6 and 1001.6 kg/m³ at 15.6 °C. The printed values are 999.0 and 1000.0 (8.35 lb/gal is 1000.5). The gap is 0.16 %, about what rounding to three decimals allows. The line is valid only from 15.6 to 100 °C. The mean expansion coefficient is printed with two significant figures, so treat the line as an engineering estimate and not as a specification.
Compared with Skydrol, HyJet IV-A plus is lighter than Skydrol LD-4 and much lighter than Skydrol 500B-4. HyJet V is close to the Eastman figure for Skydrol PE-5. See the table below. There is no density curve for the other Skydrol Type V grades, so only the 25 °C point is compared for them.