Bought first car with 1.9JTD. I decided to change oil and oil

>>17889529
You can switch to 5w at the next scheduled oil change, unless you are currently below freezing, and you'll be fine.

>>17889756
I live in a place where it's between 5C and 20C all year round.
Go for 5W then? Friend told me going into less dense oil now after running 10W40 for a long time might cause leaks or something like that. How much true is that?

Another slightly unrelated question, how often should manual transmission's oil be changed? I was told this is something you usually never do, but I have a suspicion that this can't be right. Going into 1st and 2nd gear is rough sometimes and I wonder if oil change could fix it.

>>17889784

Quick primer on oil, the first number is the weight of oil cold (as in 20°C or thereabouts). The second is the weight of the oil under normal operating temps. So, unless your car is being started in -10°, or lower, it is not going to make much of a difference. The car has always had xW-40 in it. Sometimes if an engine has blow-by (the rings are wearing out) you can reduce excessive oil consumption by running a heavier weight oil, like 20W-50.

Manual transmission oil is not commonly changed. If you have grinding gears, it is more likely worn out syncros or possibly a clutch issue. Changing fluid is not going to fix\help anything.

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>>17889784

How does your temperature only vary by 15 degrees? I live in Ontario and we get -30C and +35C every year.

>>17890659
>Changing fluid is not going to fix\help anything.

It will get rid of the metal filings that accumulate inside the transmission. Most trannies have a magnet in the bottom to collect the metal debris, but they can become demagnetized over time and get covered with debris so they're less effective.

Also the oil helps with cooling, so fresh oil had better heat dispersion than oil that's started to decay at a molecular level.

>>17890682
That applies more to auto transmissions than manuals.

>>17890742

True, but even gear oil gets dirty and loses efficacy over time. The more filings you have in the oil, the hotter it's going to get. My old Mazda only hit 130,000km before it was murdered, but I planned to change the MT oil at 160,000 as a preemptive measure. It can't hurt to change it.