RC Differential Oil Mixing Calculator
Enter two silicone oil viscosities to solve a target cSt blend, or enter a known percentage to predict the finished viscosity. The result gives scale-ready gram amounts plus approximate milliliters from the densities you enter.
Predicted blend viscosity5,000
- Oil A share
- 57.57%
- Oil B share
- 42.43%
- Oil A mass, exact
- 5.76
- Oil B mass, exact
- 4.24
- Oil A mass, scale-rounded
- 5.8
- Oil B mass, scale-rounded
- 4.2
- Oil A approximate volume
- 5.94
- Oil B approximate volume
- 4.37
- Rounded-recipe viscosity
- 4,974.28
- Mix ratio A:B by weight
- 1.36:1
Bench recipe
| Oil | Exact mass (g) | Scale mass (g) | Approx volume (mL) |
|---|---|---|---|
| Oil A | 5.76 | 5.8 | 5.94 |
| Oil B | 4.24 | 4.2 | 4.37 |
Use cSt values measured at the same reference temperature; RC oil WT labels are not interchangeable.
Volumes use the entered densities and are secondary to the scale-based mass recipe.
Scale rounding changes the estimate by -25.72 cSt from the exact recipe.
How to use this calculator
- Choose whether you want a target-viscosity recipe or the viscosity of a known blend.
- Enter the two oil viscosities in cSt, using values measured at the same reference temperature.
- In target mode, enter a target between the two oils; in known-blend mode, enter Oil A by weight.
- Set the total batch size, densities, and the resolution of the scale you will use.
- Use the scale-rounded masses for the bench recipe, then check the rounded-recipe viscosity for the practical error.
How the mixture is calculated
RC differential and shock oils are usually silicone fluids, and silicone oil blends do not behave like a simple arithmetic average. A 50/50 mix of 3,000 cSt and 10,000 cSt oil is not 6,500 cSt. This calculator uses logarithmic interpolation, which is a common bench estimate for compatible silicone fluids.
The target calculation solves for Oil A's mass fraction:
x = (ln(VT) - ln(VB)) / (ln(VA) - ln(VB))
- VA and VB are the two oil viscosities in cSt.
- VT is the target viscosity in cSt.
- x is the fraction of the batch that should be Oil A by mass.
For a known blend, the calculator skips that first step and uses the Oil A percentage you enter. The predicted mixture is then VM = exp(x * ln(VA) + (1 - x) * ln(VB)). Masses are mA = x * M and mB = (1 - x) * M, where M is the total batch mass in grams.
Why the scale matters
The exact recipe is useful, but a real bench scale can only display certain increments. After calculating the exact Oil A mass, the calculator rounds it to the selected scale resolution, keeps the total batch mass the same, and recomputes the viscosity from that practical recipe. Small batches on a 1 g or 0.1 g scale can move the final blend noticeably, especially when one oil is only a small part of the mix.
The volume numbers are secondary. They divide each exact mass by the density you enter, so they are only as accurate as those density values. When you can, weigh the oils instead of measuring by eye in a cup or syringe.
What changes the result most
The spread between the two oils matters more than the batch size. Mixing 3,000 cSt and 10,000 cSt oils gives forgiving recipe changes; mixing 1,000 cSt and 500,000 cSt oils makes each gram much more influential. Temperature also matters because cSt is temperature-dependent. The two labels should refer to the same test temperature, and WT labels should not be treated as universal cSt values because brand scales vary.
What this tool leaves out
The logarithmic rule is an empirical estimate, not a certificate for the finished fluid. It does not account for unknown additive packages, contamination from used oil, air bubbles, incomplete mixing, or the handling change a particular differential position creates in a specific RC car. Use it to make a repeatable bench recipe, then test the vehicle normally.
Worked example
Suppose Oil A is 3,000 cSt, Oil B is 10,000 cSt, and the target is 5,000 cSt for a 10 g batch. The logarithmic solve gives Oil A at 57.57% and Oil B at 42.43%, so the exact masses are 5.757 g and 4.243 g.
With both densities set to 0.97 g/mL, those masses are about 5.94 mL of Oil A and 4.37 mL of Oil B. On a 0.1 g scale, the practical recipe rounds to 5.8 g and 4.2 g, which recalculates to about 4,974 cSt. The normalized parts-by-weight ratio is 1.36:1.
Common questions
Can RC differential oil and shock oil be mixed?
They can often be mixed when they are compatible silicone fluids, especially within the same manufacturer and product line. Do not mix used, contaminated, or unknown fluids if you need a repeatable result. The calculator assumes the two oils behave as compatible silicone oils.
Why is viscosity blending not a simple average?
Viscosity changes on a nonlinear scale, so equal masses of two oils usually land closer to a logarithmic blend than to an arithmetic average. That is why a 50/50 mix of 3,000 cSt and 10,000 cSt oil predicts about 5,477 cSt, not 6,500 cSt.
Should I mix by weight or by volume?
Weight is more repeatable on a small bench scale, so the recipe is based on grams. The milliliter values are included for reference and depend on the density values you enter. If the manufacturer publishes densities, use those instead of the defaults.
Are cSt and RC oil WT interchangeable?
No. cSt is a kinematic viscosity unit, while WT labels are brand-specific hobby labels. A 40 WT oil from one brand may not match a 40 WT oil from another brand, so this calculator expects cSt values rather than WT labels.
How large a batch should I make with a 0.1 g scale?
Use a batch large enough that the smaller oil portion is several scale increments, not one tick. For many diff-oil jobs, 10 g to 20 g gives useful rounding feedback on a 0.1 g scale. If the calculator warns that the recipe rounds entirely to one oil, increase the batch size or use a finer scale.
Does temperature change the effective viscosity?
Yes. Silicone oil viscosity changes with temperature, and cSt labels are meaningful only at the reference temperature used for the measurement. Compare oils labeled at the same temperature and expect the car to feel different as the fluid warms or cools.