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Discussion Starter · #1 ·
Hey All

So I decided to pick up a quart of oil to top up my oil level in my 94 Impala SS (which I've owned since new) as I've noticed the level dropping on the dipstick. There is a slight leak on the ground that gathers and I am presuming it's from the oil pressure lines that are near the front anti sway bar. They look sludgy and they've burst once before in 1999 and I will soon have that worked on. In the meantime, as I don't want the oil too low (the dipstick shows the level near the ADD line when I checked it on a level surface with the engine cool; the owner's manual states to check it when it's warm and not running although I've always checked oil when it's cool and not running) I picked up the quart of oil (it may not need that much), however, this is the first time I've topped up my oil, on this car, whereas otherwise I've always just changed the oil normally.

What I'm not certain of is, which exact weight of oil I already have in there. I've always used Synthetic (or Semi-Synthetic possibly here and there) oil in this car since my first oil change after I bought it new. I picked up a quart (946mL) of Pennzoil Platinum Full Synthetic 10W-30. The manual says 5W-30 ideally or that 10W-30 will be fine.

I recall first using Castrol Syntec back in the 90s and for some reason I recall a crazy range of 5W-50 but now I'm not entirely sure. That being, since I'm just topping it up I'm thinking it's going to be ok if I mix weight specs (as I really don't know which range I have in there).

That said, a number of other things are running through my mind now:

-can one mix semi synthetic with full synthetic?

-can one mix a different weight range with what's already in there?

-can one mix semi synthetic (or full synthetic) with regular oil (I've seen people do that; someone I know replaces one quart with synthetic or semi synthetic with his regular oil)

-if the manual states 5W-30 or 10W-30 does that mean we should use those same ranges for semi synth or full synth?

-the car has about 80,000 miles (124,000km) on the stock LT1

-I'm thinking, even if that 5W-50 I used to use is the ideal (which it may not be in hindsight), I think that the 10W-30 full synthetic's more consistent range accuracy will protect to about at least 40 weight of what regular mineral oil would offer (however since the manual says 5 or 10W40 for mineral oil then I'm liking to think that I'm in the ballpark with Full Synth 10W-30).

-I know that the LT1s inherently run hotter (due to the reverse flow cooling) and I always remember, since new, how the temp guage would fly to the high area which is normal, however I'm not in a desert either. The summers here don't yield more than 95 degrees F (35 degrees C) so I'm thinking 50 weight is a bit of a stretch. The temps here won't dip much below 10 degrees F (-12 C) either.

so, any thoughts of my topping up with Full Synth' 10W-30 even if I don't remember what exactly I have in there?

here's the details on the Pennzoil Platinum Full Synthetic 10W-30 I bought last night from Walmart ('interesting how it's derived from Natural Gas!)


Pennzoil Platinum 10W-30 946ML | Walmart Canada

thanks everyone
 

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You'll be fine ,but stick with 5w30 synthetic in the future.
 

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You're way overthinking it. Contrary to popular myths, you can mix any oils you like. That old engine certainly won't care.

Factory fan turn on temps are pretty high. That's why temp gauge rises almost to the red sitting in traffic.
 

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Discussion Starter · #5 ·
You're way overthinking it. Contrary to popular myths, you can mix any oils you like. That old engine certainly won't care.

Factory fan turn on temps are pretty high. That's why temp gauge rises almost to the red sitting in traffic.
thanks

yes, the fan turns on way too late, which is why I often turn on the A/C for a bit, as that turns on the secondary fan immediately, cooling things down, then I turn off the A/C
 

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A few years ago I read somewhere about benefits of 10W-30 for an LT1. I gave it a try and ... every time I started the engine, it would jerk badly around the longitudinal axis of the car. Never had anything like that when the PROPER 5W-30 oil was used. So, the lesson learned.
BTW, two years ago I tried regular 5W-30 from Quaker State and can tell you my engine NEVER sounded so nice and smooth. So, from now on only Quaker State (5W-30) for my Fleetwood.
 

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Placebo effect can be strong. The difference between 10w30 and 5w30 is negligible. I ran everything from 5w30 to 20w50 in my L99 car years ago. I never noticed a difference in performance or sound.
 

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It doesn't need to make a noticeable difference to owner to make a difference to the motor.
Why is it so difficult for some to simply use oil weight(s) that GM says to use?
 

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Just to let you know. I have been using Mobil1 15-50 for well over 20 years in my Impala SS year round and I’m up to 339,000 miles now with no issues. I don’t check my oil level in between oil changes either. Also, I have the original oil cooler hoses. Heck yeah they leak but I don’t worry about them.
 

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I don’t check my oil level in between oil changes either. Also, I have the original oil cooler hoses. Heck yeah they leak but I don’t worry about them.
If you know you have oil leaks, why would you not check your oil level?
 

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Discussion Starter · #12 ·
A few years ago I read somewhere about benefits of 10W-30 for an LT1. I gave it a try and ... every time I started the engine, it would jerk badly around the longitudinal axis of the car. Never had anything like that when the PROPER 5W-30 oil was used. So, the lesson learned.
BTW, two years ago I tried regular 5W-30 from Quaker State and can tell you my engine NEVER sounded so nice and smooth. So, from now on only Quaker State (5W-30) for my Fleetwood.
wow that's quite the experience
 

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Discussion Starter · #13 ·
A few years ago I read somewhere about benefits of 10W-30 for an LT1. I gave it a try and ... every time I started the engine, it would jerk badly around the longitudinal axis of the car. Never had anything like that when the PROPER 5W-30 oil was used. So, the lesson learned.
BTW, two years ago I tried regular 5W-30 from Quaker State and can tell you my engine NEVER sounded so nice and smooth. So, from now on only Quaker State (5W-30) for my Fleetwood.
indeed, when i change the oil I'll do the 5W-30 for sure. I hope it's ok to continue 5W for an 80,000 mile motor though. I wonder, at which point, would it be better to ever use a 10W30 Synthetic, mileage wise.
 

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Discussion Starter · #15 ·
It doesn't need to make a noticeable difference to owner to make a difference to the motor.
Why is it so difficult for some to simply use oil weight(s) that GM says to use?
'some people say that if the motor is older or if the motor has a leak, the 5W may not be ideal however, I like the Keep It Simple ideology you portray, just use OEM weight range (even for synthetic)
 

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Discussion Starter · #17 ·
Just to let you know. I have been using Mobil1 15-50 for well over 20 years in my Impala SS year round and I’m up to 339,000 miles now with no issues. I don’t check my oil level in between oil changes either. Also, I have the original oil cooler hoses. Heck yeah they leak but I don’t worry about them.
is that 15W50 Synthetic? that's awesome. lotsa awesome reliable miles!

yea, my oil lines, where they're leaking, have burst before, so no WOT until I get them redone, just in case
 

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Belair, yes Sir.
That is Mobil1 15w50 Synthetic. Been using since 2000 year round. The Impala is at 339,120 I run up and down I-35 mostly highway. I do my oil changes between 7,000 to 10,000 miles. I’m sure some people will flip out but each their own. I usually put in 5.25 qts at oil change. I never check oil dip stick level in between oil changes. The only thing I’ve replaced on the engine is starter, belt, oil and filter, air filter, spark plugs, plug wires, ignition cap, rotor, radiator cap, water pump and coolant hoses. The alternator is still original. This Impala is the best car I have ever owned.
 

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Oh, I forgot the Right exhaust manifold was replaced to. It was cracked all the way around under the heat shielding. Other than those items the engine has never been apart. Also the original opti-spark distributor has never been removed from the engine except for the cap and rotor. Left exhaust manifold has a broken bolt in the very back but it doesn’t leak exhaust. This thing does not smoke out the exhaust at all. It’s unbelievable GM nailed this car. My only complaint is the crappy plastic interior. Every piece of plastic in the car is broken and brittle and faded from the sun, cold and heat. 339,000 miles is crazy and it keeps going strong. What more can you expect from a 25 year old car !!
 
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