replacing the gearbox oil with fully synthetic
Personally speaking mate , my guess is your be wasting time and money . Your gearbox like mine , is built for mineral oil , not synthetic , so it's highly likely your find the problem far worse . Certainly shorten the life of the gearbox , though from what your saying i seriously doubt that's even possible . I will admit right now , i'm a little outta my depth here , but i'll try and help where i can . I've never had to work on any gearbox !
So if i may , i'll recite a little incident from a few years ago before i get started . Back in the 90's , i was given a 70's Japanese motorcycle . Now though it started and ran reasonable well , it used oil like it was going outta fashion , and could lay smoke better than any 2 stroke of the era . Not bad for a 4 stroke huh . Quizzing the previous owner , i discovered he was using a fully synthetic racing oil , specifically formulated for motorcycles . Now i knew this machine was not built for this , as i'd had one years previously . So i drained the oil , flushed the system , cleaned the filter which was not the replaceable type , and refilled with a high grade , 15/40 multigrade . It transformed the bike , it stopped smoking instantly , and the machine never used oil again . It had grip problems as well , but thats off subject .
Now one thought does spring to mind here , in that you are sure the gearbox is adjusted correctly arn't you . The linkage between the shifter and actual selector on the gearbox is by cable . I can't really help here , as i've never had to touch mine , but i could probably find the sequence and pass it on if needed
As for synthetic oil , i would not recommend it , i couldn't even guess at what synthetic oil alternate you could use . However there are a couple of other things you could try , but i'll be honest i have no idea what the outcome will be . First off you could try a different grade gearbox oil . My van is currently off road having some major restoration and upgrade work done , so i've not changed the gearbox oil in a while . Now if i remember correctly , my van requires a 75/80 grade gearbox oil . Your van is a year older than mine , so my guess is you should be the same . You could try upgrading to 85/90 , but be aware , if you have the ME5TU gearbox fitted , which due to the age of your van is likely , you could very well shorten the 5th gear life , as lubrication may well be reduced . Quick way to tell is from the shifter . Sat in the seat , reverse is far right and down , under 5th . Far left and up is the ME5GU , which doesn't suffer from the same issue .
Now the second is something i would advise you master as soon as possible , and its a driving technique called double declutching . This is a method of allowing the revs and speed to equalise prior to a gear change . Basically exactly what a synchromesh does . It's not difficult to learn , but is difficult to master , however once mastered a gear change can be both slick , without using the clutch , and quick , matching the speed of any clutch change . I don't know if you've ever being taught this driving technique , but basically it's a deliberate shift and pause in neutral , release the clutch wait a second , then re-engage the clutch and gently engage the next gear . If the rev's and speed match the gear should slip in , if not you will find stiff resistance . The problem is every gearbox is different , but even so revs should be around 1000-1500 , you just need to find that sweet spot . There is a slight difference going down , as a gentle rev is used whilst in neutral with the clutch released to pick the revs up , then clutch down , and select the lower gear .
So as a recap , changing up , clutch down , shift to neutral , clutch up , wait , clutch down , select . Changing down , clutch down , neutral , clutch up , rev , clutch down , select . Got it , practice , this will do more good than anything else .
I'm sorry , but from what your describing , my opinion is the only way to resolve this issue completely would be to either repair the existing gearbox , or replace . However i do understand your wanting to make the thing last , more so in this current climate we find ourselves in right now , but in my opinion , anything else is simply gonna be a waste of money .
