Hi
Today i have changed the rear diff oil, i use Castrol Syntrax 75w140 Limited Slip, the old oil was black, it was unusual to find that.

Interesting, after the oil has been changed and drove the car to home i found that the car consumption in the city from 19.1l/km reached 18.3 (15.4 mpg UK gallon) and all this in 8 km.

Any one notice an improvement after the diff oil was changed?

 

In theory old oil would be dirty thus affecting it's tension w/ debris, making parts moving through it labour harder I suppose. Clean = less work & in the end use less resources.

 

I'd hold off getting too excited until you've driven farther. So many little things can effect the readout of the mileage, based on the time-frame it's being calculated into. That said, as KC mentioned, if there was excessive buildup where it was basically more work to spin through the sludge, then it would make sense to get slightly better mileage when it's fresh oil. Just like an engine will rev freer if you put new oil in versus very old stuff that's all gunked up.

 

The energy created by the engine would have to go somewhere. If the oil was preventing the energy from reaching the rear wheels through addtioanl friction, the amount of heat created would cause the diff to glow red hot.

 

Maybe not red hot, but point well taken. Fuel consumption measured over such a short distance is not relevant.

 

I do not reset the computer, before to change the oil when i start the car in the morning the OBD show me 19.1 to 19.4 and stop after the engine reach the temerature at 19.1. Yestarday this value decrease for the first time under 19l/km.

A feaw months ago i search on the forum to see other cars cunsumption, most of the people say that XJR make 15 to 17 mpg in the city, my value was 14mpg or under in the winter.
I do not change the oil on the diff with the consumption in mind.

In a few days i will change the filter and transmission oil, i will check after for inprovements.
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