I just bought an '88 GTA with 113k. I did a rear end service and cleaned out all the old gunk and fluid and metal shavings. I refilled it with Castrol 75-90 Synthetic gear oil and limited slip additive. It seems to be fine despite the fact that the old rear end most likely had convetional gear oil that had never been changed. My big question is this: What if I convert it to Castrol Syntec 10-30 in the crank case. I just did an oil change last week and put in the GTX, but I use Syntec in my other two cars, so I'd like to change over if it won't hurt anything. I know that they say if you have any oil leaks not to do this because it can make them worse due to the smaller molecules. The only know oil leak was the rear main seal but I just put a tranny in two days ago and I replaced the rear main seal while I could get to it. The car smokes a little blue on a cold start, but it immediately stops after a few seconds. I figure this is the valve stem seals. Is there any known issue with making this change over on an application like mine? Could it make he stem seal leak worse into the chambers? I wanna do this car right, so let me know thirgen experts, Thanks!!!

 

i am running 15-50 in my car, 130 tho all conven oil before that, no problems here, on my 2nd oil change with moble1

 

If the rear main was leaking it is likely that switching to synthetic may cause more leaks. It may not happen after the first or second change, but it could happen later on. It just depends how much crap there is built up inside. I've known a few people with close mileage that have had problems, myself included. Changing should not make the valve stem leak worse, if anything you will not notice it as much because synthetics tend to burn a bit clearer than normal oils.

 

I'll admit my bias up front, as I have no love lost for the synthetics put out by Castrol, Pennzoil, Quaker State, or Valvoline.

But, I changed my 123k engine over to AMSOIL and had the blue smoke problem and high oil consumption. I replaced the valve seals on intakes only, helped the smoke problem a lot and took care of the consumption, but I really should have done the exhausts while I was at it (still some smoke with certain hot start-up conditions). A slight weep developed around the oil pan that lasted for about a year - the pan would get wet but not drip. By the time I did my sig mods at 143k, the pan was dry.

I'd say do the valve seals (all 16), use something better than Castrol (like AMSOIL, Mobil 1, or Redline), and you'll be fine.

 

This really depends on the condition of your engine and how you use the car. If the engine is in good shape, has been well maintained with regular oil changes and has limited wear, you can probably go to a synthetic. However, if the engine has significant wear, you may need to stick with Dino oil. As you mentioned, synthetics have smaller molecules, and therefore tend to squeeze past the larger gaps in a well used motor and you burn more oil. Also, if the engine is in a weekend warrior, you may want to stick with Dino oil. Synthetics, because of their smaller molecules, have a tendancy to run off parts more quickly while Dino oil has a tendancy to stick to parts making infrequent start ups a little less traumatic. I'm not saying Dino is better than synthetics. It just depends on the condition and use of the car. In your case, it sounds like the rear end was not maintined well, so I'd be suspicious of the engine. Also, it could make you valve stem seal problem worse. Also, if it wasn't burning oil before, it may start.

That's my $0.02 anyway.

 

Thanks for the input so far guys! I think I may decide on keeping the convetional oil for now because I don't wanna risk anything. It is a second car, so I will not be driving it all the time. (I have a 350 on the stand in the garage waiting for when this L98 is dead, I will start it out fresh on sythetic after break in once I drop it in the car). No offense to Redline, Amsoil, or Mobile 1, but I have a reason for my faith in Castrol, and it is all I will use. These other oils are definitley not in question though, I just use Castrol because it is readily availible and since I've had succes with it, why change over? Also, Royal Purple is another worth mentioning in that bunch. Incidently, my Jeep leaks a tad and it has had synthetic since for the past 20k.

Phase II: What negative effects can puting the synthetic in the 113K Borg Warner rear end have? It looked solid and clean when I was in there(once I removed the old fluid). It is a tight rear end, posi works good, and no whine or chatter. I have heard that it may not be good to switch to synthetic in a gear box with eccesive wear, but this one seems okay. Seald don't leak either. Am I okay??? In my Camaro, I used conventional cause it had high miles and lots of wear on the pinion gear.

 

TBI,

Synthetic lubricant (not "oil") in your differential shouldn't cause any problems. As long as you added the correct limited slip modifier (as you stated you did) the axle should be better off with the fresh lube.

I have had problems changing a manual transmission to synthetic, since the lubrication was so good that the synchronizers would slip too much and not allow smooth downshifting. I went back to less-effective mineral oil and the synchros started working again. Strange, I know.

As for the synthetic in your engine, as long as you repaired the rear main and have no leaks, the change can only be a change for the better. My TA presents the "blue salute" at every cold start, and has done so since it had 18,000 miles. I use synthetic religiously, and have no problems other than the crappy factory valve seals that have hardened. I'm not sure of the properties of Castrol, but Amsoil and Mobil 1 have seal conditioners that may eventually actually help a leaking engine as long as the seals are not damamged. Many times it is only the dirt and varnish around seals and shafts that cause the leaks, and once it has been cleaned away the seals can once again do their job.