The manual does not specify when the differential gear oil be replaced. It only specifies to inspect at every 15K miles. How often do I need to replace this oil and what kind of oil to replace with?

I replace mine every year (about 15-20K) with my yearly flushes... That may be overkill, but oil is cheap compared to parts. I use Mobil 1 in whatever grade the owner's manual calls for (50w-90??) I can't recall at the moment.

Heath

I also replace every January,along with all the other fluids.Redline,Mobil 1, and even Amsoil are all excellent full synthetic gear oils to use.

Recommended viscosity is 80W-90 as long as ambient temperature is below 104*F.

Its been about 5k since I did mine last.....

I say about 10-15-20k is a good interval. For the amount of time it takes and the cost, it should be done more.....

Thanks guys!Wow! Glad that I asked this question, because I have never changed the differential gear oil on the five cars that I owned.

I have never changed the rear oil either....although I keep threatening to do it.

The cooler weather is coming...maybe then...

Today, I at least cleaned the TB and MAF and added a can of BG44k to the 94

»How often do I need to replace this oil and what kind of oil to replace with?

I'm no mechanical engineer, but, it may be extremely important to note for the Nico users that stating just the SAE viscosity for gear (or motor) lubrication is telling only half the story.

Due to the fact that differential gear planes are often offset from the centerline (i.e., hypoid) which places unbelievable stress on the gear teeth (over that of a straight or spiral bevel gear), I believe the API "quality" (or performance level) is even more important for hypoid bevel differential gear oil than it is for combustion engine oil.

For example, my 1990 Infiniti Q45 factory shop manual states (page MA-9) the API quality for the differential gear oil should be API GL-5, with a "preferable" viscosity (page MA-10) of SAE 80W-90 (depending on ambient temperature, as noted) and a capacity of 3 1/8 pints (page MA-19).

Once you have API GL-5 or better, of the desired viscosity, I suspect the price, the brand, and even the particular composition is not meaningful (unless experience dictates otherwise). My main point is, don't put in API GL-1 or API GL-4 gear oil of any brand! Use API GL-5 (MIL-L-2105D) or better.

Why not GL-4? I wondered the difference myself, so, I looked it up. For example, this lubrication PDF (http://home.earthlink.net/~cmc...7.htm) warns "a typical GL-5 will contain approximately twice the amount of Extreme Pressure (EP) additive as that of an API GL-4 gear oil" which is important for hypoid gears.

I wasn't sure if differential oils allow the substitution use of the next-highest API value like motor oils do as noted in the API publication http://api-ep.api.org/filelibr...2.pdf so I also looked it up and found this interesting PDF http://api-ep.api.org/filelibrary/1560.pdf which confirms that the API GL-4 service classification is intended only for non-hypoid gears while GL-5 can cover hypoid geared differentials as well.

Interestingly, I didn't see any mention in the 90 FSM whether the Q45 differential actually contains hypoid bevel gearing, nor did any diagram show the actual gears, but I suspect it does based on the requirement for nothing less than the API GL-5 service classification.

Can anyone confirm whether the Q45 actually uses hypoid gearing in the differential?

tangalora wrote: My main point is, don't put in API GL-1 or API GL-4 gear oil of any brand! Use API GL-5

That's very true.. thanks for adding that!

Heath

Thanks for adding "don't use API GL-1 or API GL-4 gear oil of any brand! Use API GL-5".

I'm always glad to assist in the very little that I can (I've been helped greatly by this Infiniti on-line mechanics' forum and I'm a give-a-little take-a-little you-help-me I-help-you kind of person).

Normally I buy the best measured & proven "quality" or "service" for the lowest possible price (caring little to nothing for particular market-branding shenanigans that I can identify such as Chevron's moronic techron-based preference-shifting advertisements).

Usable measured & proven quality for some items (e.g., brake friction materials, oil-pan-repair pastes, aftermarket shocks, etc.) is miserable or impossible to obtain (as noted in prior NICO Q45 brake-repair & oil-pan-repair threads); yet, luckily, for other common items (e.g., gasoline, lubricants, tires, etc.), it's quite easy to ascertain by the lowly buyer (me) who shuns brand names & higher-than necessary prices. Unless I know better, I simply aim for the best quality I can afford -- knowing full well price has nothing to do, per se, with quality, (as can be witnessed by Joe of Scottsdale's expert yet much less expensive OEM parts-delivery philosophy or this free yet phenominal use of the NICO forum). In my opinion, you (almost) never get what you pay for. Despite the old adage, you simply get what you get. Period.

Therefore, with that in mind, given that GL-5 is required by Infiniti for differential gear lubricants, I'd have searched for GL6 (or better) after taking the price (yet not the brand, unless I knew better) into consideration. That is, if some extreme pressure protection is good, more should have been better; so I'd have recommended (in vain) that TorQue45 consider pouring API GL6 gear oil into his Q45 differential if he wanted better protection than GL5 affords.

However, one thing that amazed me in researching API service classifications for gear lubricants yesterday was the fact that API-GL6 gear oil, while admittedly superior in extreme pressure situations to those of lower GL-numerical order (i.e., GL5, GL4, & GL1), is (apparently) now considered to be an obselescent service classification (along with GL2 & GL3).

Looking up why, it appears the loss of API GL6 tested & designated gear oils seems to have both practical & theoretical implications.That (apparent) lack of API GL6 oil on the market is explained in the aforementioned API PUBLICATION 1560 SEVENTH EDITION, JULY 1995 publication titled "Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles" http://api-ep.api.org/filelibrary/1560.pdfThe designation API GL-6 denotes lubricants intended for gears designed with a very high pinion offset. Such designs typically require protection from gear scoring in excess of that provided by API GL-5 gear oils. A shift to more modest pinion offsets and the obsolescence of original API GL-6 test equipment and procedures have greatly reduced the commercial use of API GL-6 gear lubricants.

Which leaves us with the (now re-summarized & repeated) NICO recommendation for TorQue45 to use API GL5 (or GL6 if it can be found) but never to use GL1 or GL4 gear lubricant of any brand or price in his Q45 rear differential.

I own a q45 1998 and just put gl 5 pennzoil in the differential reccomended by the dealer. Can anyone help me with its performance....is it good?

Should be fine... keeping up with the frequency of the oil changes is probably more important than the particular oil you use, as long as it's a decent reputable oil.

Heath

vicentelicona wrote:I own a q45 1998 and just put gl 5 pennzoil in the differential reccomended by the dealer. Can anyone help me with its performance....is it good?

I would be more concerned with the previous 14 years of maintenance history of differential fluid changes by previous owners than whether your brand new fluid is "any good".........even if you put in the very best pure synthetic $15/quart gear oil now......the previous fluid change routines over the last 14 years of use,if any, is what is going to ultimately determine the longevity of the differential........too late now to "un-do" any lack of maintenance and therefore wear by worrying about using the "best" fluids at this point.