It's Getting Colder - Estimating Rate of Heat Transfer Across Boundaries

It depends a lot on how much oil (especially how thick) is on the other side. No simple single rule is likely to suffice, because it also depends on the thermal conductivity and turbulence of each fluid. In the "simple" formula Q = KA(lmtd), there is lots of devil-detail in K, the thermal conductivity factor.

It depends upon the cooling capacity of the thing that is generating thermal oil at -50degC. If it can't keep up then it will take forever.

Is the 20l/m being recirculated or is it going somewhere else?

If you don't know the thermal conductivity of either of the fluids or the separator plate what good will formulae do you?

Besides this sounds like homework.

I convection mode of heat transfer heat is carried away from the wall surface by the flowing fluid. Convective heat transfer can be either forced type or free or natural type. Heat transfer coefficient is very important to arrive the heat transfer rate.Q = hA TD. H is the heat transfer coefficient and T.D is the temp difference. If h value is not available we cannot complete the calculation