Here is my experience on the oil burning issue on my i35 and my steps to corrective action:
My car: i35 2004 with VQ35.
Current mileage 125k. issue started at around 75k
I did a whole lot of research in the maxima forums, bitog and other infiniti forums for various solutions and the following helped.
What i believe: The oil consumption issue in the VQ35 is caused by the following:
a) Bad piston rings
b) bad valves
c) Oil leak in the Right Valve cover
d) PCV valve frozen ( but why ? see later)
e) Rear main oil seal ( infrequent)
Majority of the oil burn cases are a) c) b) d) in that order
At about 75k , i started burning a quart in about 800-900 miles. Shocked i started researching. Tried various weights of oils and brands and finally found that the following worked best on my car
Penzoil Ultra 5w-30 or 10w-30 in summer. Provides the least burn and moved by burn cycle to 1800 miles per quart.
I think PP also works fine , but never tried
What has happened to the rings ?
Bad design has resulted in the oil rings not being a tight fit / not cleaning up properly. This results in blowby which further causes the ring to accumulate carbon deposits. Over period of time the rings are fully packed with carbon deposits and largely fail to expand contract as needed. This causes higher blowby and heat into the crankcase which translates to the pcv vale pumping out more vaporized oil through the intake manifold. Your oil also turns black faster, by 1500 miles it has turned darker.
At this point oil is making its way into the engine via two routes: pcv-valve->Intake manifold and through the oil rings itself since they are largely frozen.
An example with pictures can be found below for the VK45 engine which suffers from the exact same issue.http://www.infinitifx.org/viewtopic.php?f=2&t=25605
We need two solutions to counter the bad design:
a) Clean the rings
b) prevent the rings from clogging up.
c) trap any possible oil in the intake manifold.
a) Clean the rings: There are several ways to clean the rings ( MMO soak, seafoam etc and numerous others). The two most effective ways that worked for me are as follows:
i) Do the KREEN treatment. drop kreen into the crankcase and fuel as directed ( dosage) and then run for 900-1000 miles. Do not extend oil with kreen in it for more than than recommended. kreen eats away varnish and gunk from the engine. Keep it too long and it will eat away gasket seals. After replacing Kreen check your valve covers and tighten the nuts. In my case oil was leaking out of the valve covers after the Kreen treatment and that also created a vaccum leak. I tightened all the bolts for the valve covers and that fixed the leak issue
ii) Replace oil and at the end of the next cycle run a BG109 ring cleaner in the engine and change oil. Follow instructions and do not overrun the engine. Instructions are to run for 10 mins in neutral at 1200 rpm and then flush. Follow the instructions !. play safe run for 9 min and quit.
III) Replace oil and filter ( penzoil 5w-30 with mobil 1 M110 filter)
Drive the car and you will see that the engine revs up very smoothly to 5k ( both standing / on the freeway). No sluggishness.
b) Preventing the rings from being clogged again:
Drop TCw-3 two stroke marine oil ( 1oz per 5 gal fuel into the tank). This lubricates Upper cylinder engine and may give you a mileage boost as well about 1-2mpg. The TCW-3 is clean burning and does not leave any residue. The particular one that i use is the
Penzoil Marine Premium Plus Synthetic Blend 2 Stroke TCW-3 Oil. A gallon goes for about $16 in the local auto stores and should last about 40 full tank ( mytank is 16 Gallon). The most important thing here is the TCw-3 labeling.
What does TCw-3 do ? Since it is in the fuel it helps lubricate the upper cylinder and the valves + being clean burn burns away any residue. It is also designed to prevent piston rings from sticking in the marine two stroke engines (2 strokes ahve no oil other than what is mixed in the fuel for lubrication). In the lower dosages it works wonders on the 4 stroke engines.
I carry a six pack 3 oz bottles filled with tcw-3 in my trunk. every full tank i drop a bottle in just before fillup . This is something you will have to do all the time. the six pack lasts me 6 weeks. You can go to wallmart and buy 3 oz travel bottles in teh travel section for 99cents each.
It will take about 2 full tanks to see the effect.
c) I added a catch can : http://www.42draftdesigns.com/categories/products/catchcans.html
Please do not install the cheapo catch cans. this one has special sieve to allow oil to deposit when vaporized.
Oil burn is now down to a quart at about 2400-3000 miles. I basically add a quart about 2k miles and then go for an oil change at 5k. BTW i start with a full 5 quarts in the engine.
Engine Ping completely eliminated. ( caused by intake oil seep).
Even at 5k oil is not black on the dipstick
Mileage on the car went up by 1 mpg ( 24 in winter, 26 in summer).
Engine response is amazing.
Bad rings are not going to solve the oil burn completely unless you rering them. However this process is largely unintrusive and should take me to the 200k mark with ease with a little attention to the oil and the fuel mix.
On the oil front: PP Ultra 5w-30 is an amazing cleaner. I was an M1 fan for 20 years. I have switched to PP ultra at thi spoint. Not easy but the results convinced me otherwise. GC resulted in 1-2mpg lower mileage.
What else had i tried before this conclusion:
1) Replaced plugs
2) Cleaned MAF
3) Tried GC 0w-30, M1 0w-40 ( being heavier both burned less, but not by that much)
4) tried MMO in the oil and the tank ( helped some)
5) Tried the usual gas cleaners ( techron etc)
6) replaced PCV valve
7) checked for Right valve cover oil leaks and spark plug leaks ( mine were dry).
I have reached this conclusion after trial and error ( systematically) over 50k miles. Hope this helps. YMMV.