Noise of the 5th gear is a common problem with Khyber and Cultus (both have the same gearbox). Though I don't know its solution, it is always best to bear with the noise as is, since if you open up the gearbox, you'll have a terrible time finding the proper sealant which would work for a long time. Usually our mechanics use Bossil RTV (Bossil is a Malaysian brand), gear oil eats it quickly and then you have a leaking gearbox.
Also, do not use 140 weight gear oil is modern FWD gearboxes. Now you would ask Cultus is not a modern technology. By modern I mean anything from the last 30 years.
Remember to use GL-4 gear oil in FWD gear box. GL-5 is EP (extreme pressure) oil and only suitable for differential in RWD cars. Even in RWD cars, for the gearbox use GL-4, and for the differential use GL-5.
If you look at the labels, you will find that mostly GL-5 oils are heavy weight and GL-4 are lighter.
In the GL-4 oils, I found ZIC gear FF to be good. The gear shift doesn't feel heavy at all even when you shift it in 3-4 degrees centigrade. With all the other oils, PSO etc. I found that winter driving is difficult in the morning, even when the viscosity used is same (like 75W-85).
NOTE: ZIC gear FF is synthetic oil, thinner than most others and also twice as expensive. But it is something you have to change only once in a while, its worth the expense.
NOTE: The viscosity grade of ZIC gear FF (or any other oil e.g PSO gear) may fall slightly different from that specified in the owner's manual - but manual gearboxes are extremely tough. It is better to put in some reliable liquid which is 19-20 rather than some unknown brand which could be exact on the label but no surety what it is actually. Also, synthetic oils have very consistent performance over a wide range of environmental conditions.
NOTE: In my experience with old gearboxes (~20 years old), means gearboxes with old rubber, ZIC gear FF caused sweating on the rubber seals of gearbox. But no big leakage. Droplets weren't noted on ground.
ABOUT REFILLING FROM THE SPEEDOMETER (VSS):
Most mechanics out of laziness fill by removing the speedometer unit and putting a funnel there. This will sure result in overfilling and soon the seals would leak/burst and you will be pulling out the gearbox the replace the seals (which means additional time and money). Best way is to use the leveling nut. You do NOT need to pour in any more oil than required. Manual gearboxes do not have an oil pump like the engine (automatics do have - at least some). Oil is lifted by the gears (cogs) and thus gets on each part where it is required. It is not required to submerge in oil every part of the gearbox just like an aquarium.
Remember - with any machine - a little less oil is acceptable (as far as it is above the minimum mark) but even a little overfill is going to cause many problems (leaking seals, reduced machine life due to oil frothing etc.).