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So my mini was running smoothly, but Two days ago is slowing down and really suffering when taking up hills

The only change I made recently is the fanbelt, so I wonder if the problem could be that I didnt tighten the fanbelt correctly or something the sort?

Does it get hotter have you noticed? Oil pressure OK?

Off the top of my head the belt change shouldn't make any difference, and I seem to recall you found the right one?

First thoughts my end would be timing and or fueling, so I'd check timing first and then consider richening up the mixture.

If there is anywhere nearby, nothing beats a decent rolling road.

Fingers crossed for you.

Often overlooked: Is the throttle opening all the way?

Well it doesnt heat up but when stopping for traffic lights the oil pressure indícator does blink once or twice, does it need some more oil?

Thanx

Check the dipstick !

Ok, so I checked the oil level and it was half empty and black as petroleum. So the past weekend when I changed the oil alltogether I realised the engine was an oily mess, specially in the bottom parts, so could the leaking be due to the fan belt change?, cause is the only thing I did recently.

Also, I´ve been told that for this car (mk1 850) the best oil is 40 monograde but in several tutorial videos they recommend 20w 50.

thanx

An oil leak doesn't slow the car, you should check your rocker gaps and if the bushes are worn. Do a compression test from cold and then another when the engine is hot. You may need a ring job or/and reborn. Good luck. R

20/50 is the correct oil for Minis, not a mono grade.

Frequent changes are the key to longevity

Whatever else is going on Nick, that oil leak needs seeing to sooner rather than later I'd say

Ho dear, the most critical aspect of engine oils is the add mix of property enhancers. Mini drivetrain ' needs' high percentage of ZDDP. Zinc dithiophosphate, which combats sheer. However this is not helpful in cat equipped cars, so is not generally part of the mix today. The characteristic which is far more significant is the API rating the product carries. But you need to look at the back label to find it. Looking at historic data it seems that API SC, SD, SE grades would be appropriate for a mimi transmission. Looking just at the viscosity numbers just informs how the fluid,be that water, honey or oil behaves at certain temperatures. A multi grade simply uses chemistry to artificially make the fluid runnier than it would normally be at cold temperatures_ so that the weedy little battery and starter can actually rotate the crank up to lift off speed, about 150 rpm. If you are a typical owner not using your car between Halloween and drive it day, it doesn't matter a bit about the oils 20W performance so yes using a straight grade is a no braner, provided it has the right API grade. If you are in the UK tonight when we are expecting a shocking -5 centigrade then you will benefit from the 20w tomorrow morning for ten minutes after firing up.