Idemitsu – Type H Automatic Transmission Fluid, 1 Quart (Part No. 10116-042) Engineered for and meets requirements of Honda Automatic Transmission with Z-1 specifications.

Lubegard Synthetic Synthetic Transmission Fluid Dexron VI 1 Quart.

If Honda MTF is not available, you may use an SAE 10W-30 or 10W-40 viscosity motor oil with the API Certification seal that says ”FOR GASOLINE ENGINES” as a temporary replacement. However, motor oil does not contain the proper additives, and continued use can cause stiffer shifting.

Unlike the automatic however, where you actually have to flush the fluids with a machine for preventative maintenance. The manual requires a simple, in comparison, drain and fill of the transmission fluid. Most manufacturers recommend that manual transmission fluid be changed every 30,000 to 60,000 miles.

The simplest answer to the difference between transmission fluid and gear oil is in the purpose for which they are designed. Transmission fluid is engineered to work with automatic transmissions, whereas gear oil is typically intended to work with manual-type gear boxes.

4. Slipping Gears. A healthy transmission will shift between gears smoothly and there won’t be any slippage. Low fluid levels can cause gears to miss, which will result in a grinding sensation.

Whilst some manual gearboxes do use transmission fluids in place of gear oil, the same cannot be said for putting gear oil into an automatic transmission. Gear oil is much too viscous for these systems and will cause clogging between components.

A 75W-90 will be thinner at higher oil temperatures than a 75W-140. Both are the same thickness at lower oil temperatures. The 75W-140 is said to have a wider viscosity range – when more accurately it would be a wider temperature range.

After selecting the viscosity grade, the basic type of lubricant must be chosen. While there are many variations, gear lubricants can generally be placed into three categories: R & O, antiscuff and compounded. The gear lubricant type that best fits a given application will be determined by the operating conditions.

Gear oil and hydraulic oil are two different fluids that come under the category of lubricants. There are various types and combinations of hydraulic fluids and gear oils either in the form of mineral oils or synthetic materials, all blended with additives.

The simple answer: Yes. There is no danger mixing synthetic and conventional motor oil; however, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits. So, yes, you can safely mix synthetic and conventional oil.

Synthetic lubricants as a class don’t show their age, particularly at high temperatures, and have a longer service life. Synthetic lubricants have a lower friction coefficient in a gearbox, better film strength and a better relationship between viscosity and temperature (viscosity index, VI).

You are fine to mix the two I’d think. No weird additives, just basic Castrol 75w90.