was told something like this but cant afford to buy 20 ltr

the 5w-30 variant is the one to go for (not because of it being slightly thinner at room temp, but because it has a different additive package which makes it better for manual transmissios with synchro rings, so that you can optimally offset the cost of having to buy it in larger quantities (they might do smaller than 20 litres, but I'm not sure) by being able to use it as gearbox oil as well.