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Mild grabbing/grinding when downshifting after gear oil change
I have a 4wd 5 speed manual 1986 pickup with 140,000 miles. I just drained all my transmission oil and replaced with this Lucas product. I also added about 1/4 quart of the Lucas Oil treatment (the thick stuff) it states on the side of the bottle that it can be added to manual transmission oil. I don't know what kind of oil was in the tranny previous as the truck is new to me. I filled til oil was flowing out of the the fill hole on the side of the transmission and then capped it. I forget exactly how much it took to fill but I think it was between 4 and 5 quarts.
It now shifts less smooth. All gears are acceptable except for second. When I downshift to second I have to try to do it quickly but even then it feels forced and I feel a slight bit of grab/mild grinding. The gear oil I added says that it is gl4 and gl5 compatible. I thought for sure I was using proper gear oil. What do you think?
There are likely to be a number of differing opinions on which oils are best.
Sometimes the GL-5 rated oils are a little too slick.
Also, thicker is not always better,especially for colder weather service.
Generally, the newer spec gear oils (GL-5) give excellent service in differentials, but older tranny syncros sometimes don't care for them.
I use synthetic GL-5 gear oils in my diffs and transfer cases, but i buy a semi-synthetic Euro-spec 75W-90 GL-4 only (not GL-4-GL-5) gear oil for use in my trannys. This is the oil that contains the additive package that the older trannys were designed to use. Mine do well on this oil.
I can get it for about $60 for a 5 gallon pail from the oil distributor in my area, so I can change oils out for little cost.
Manual transmissions have somewhat different lubrication requirements than do rear ends/differentials. The syncros in the manual require friction to work - they're essentially clutches that force the gears to match speeds, so introducing oil and/or additives that are super slick will cause the syncros to not work well with resultant shifting problems.
On the other hand, the hypoid ring and pinion in a rear end is actually closer to a worm gear drive than a straight right-angle gear. That means there is a lot of slipping going on between the teeth, requiring oils that are slick and that can maintain their film under very high pressures. GL-5 oils are designed for this.
Millball is correct, I think you'll do better with a straight GL-4 in the tranny, and skip the additives unless you're an oil chemist and know what you're doing.
I'd try Redline mt90. My trans was having similar issues at one point after a rebuild and a simple change in gear oil type is actually what resolved it.
Now that I'm thinking back, I think I added 1 or more quarts of Mobile 1 gear oil along with the Lucas. I just looked at the bottles and the Mobile 1 is designated as GL5 only. The Lucas, which was the majority of what I added, was GL4/GL5 but the Mobile 1 was not. I was a bit short and added the Mobile one, which I originally bought for the differential gears. I just didn't remember that it was GL5 only. This may be my problem. Both were synthetic and 75w90 though. Quite a waste of money, but I think I'm gonna flush it all and start again.
I switched to the Redline MT-90 and was disappointed at first because the symptoms were exactly the same. But throughout the day as I drove the truck things smoothed out. It is noticeably better now. I guess using a straight gl4 gear oil really is the way to go on these trucks. I now have 4.5 quarts of great looking synthetic gl4/gl5 lucas gear oil that I drained from the tranny into a bucket. Will have to find another use for it.
It's interesting because before when I first changed the gear oil I didn't notice a huge improvement or difference and then throughout the next day or two shifting became more rough and grabby with the occasional mild grind on downshift. I wonder why it takes a bit of driving before the new gear oil takes full effect? Perhaps the oil has to make it's way into little areas and coat certain parts.
I agree with the Redline MT90. I also agree to give it 200 miles or so to notice more improvement. I haven't double clutched much as millball said, but that stands to reason. I do hesitate when possible, start into the gate and wait for it to "catch", goes in gear better. You can feel it.
09-23-2015, 08:19 AM
