Hello there,
I'm the happy owner of a 1986 C4 with L98. I live in the EU so it's not easy for me to get access to spare parts. hence i would like to maximize as much as possible the good state of its engine, starting by putting the right oil.
I would like to kindly ask you guys which oil do you recommend for L98 (cast iron), it has around 79k miles and is running fine afaik.
I know the previous owners run 15W40 and 10W40, with the latter currently filled in, while the manual recommends 5W30.

I read a lot on the forum but i'm unable to understand the final point. There are a lot of people recommending Mobil 1, but i would like to find the correct one here in Europe. I attached a sample of what's available (5W30).
Is there any rule of thumb for choosing the oil? Should i look for API SN, SM, SL or Dexos 2 specs and are those specs enough to be considered compatible?

Thank you!

 

Probably 3 answers for the heavier oil.
1. The engine has a slight knock with thinner oil.
2. The previous owner lived in an extremely hot environment.
3. Recommended diesel oil has high pressure lubricant additives.

oil that is a multi weight oil is actually the thinner oil(5w-30)
the additive is a thickener reacting to heat.
so when the oil is cold, it is 5 weight, and the thickener increases the weight when hot.
any 5w-30 is acceptable since you are not racing the car. Mobil 1 is synthetic, but any oil is acceptable with one condition.
in 1986, GM was maturing the corvette engine, and they had got the one piece seal done, and the right hand dipstick, but was still researching and perfecting the roller cam for production engines.
if your engine has iron heads, it has a flat tappet cam.
you need to run an additive zddp.
because that additive destroyed catalytic converters, the US government asked the us oil companies to remove the high pressure lubricants from the oil that is sold in the US.
synthetic oil is so slippery that it becomes the high pressure lubricant that protects the camshaft from exaggerated wear.
without additives or synthetic oil, your cam will go flat.
i bet the previous owner thought if he ran diesel oil, it would protect the cam since it has the high pressure lubricant in the oil, and it’s cheaper.

 

Thank you coupe guy for the answer and the explanation!
I believe the heavier oil could be only due to knocking, or maybe oil leaks. I'm more for the latter since the engine is leaking a very small amount of oil mainly from, I suspect, the driver side valve cover gasket as the engine is oil smeared on the outside and i find some (few) drops of oil under the car. So the thicker the oil, the lesser the leak as far as I know. Anyway I'm not yet worried as so far i found no sign oil combustion (quantity is ok, no colored smoke, etc).

Long story short, if I understood your point correctly any full synthetic 5W30 should work as it lubricate correctly the engine AND the camshaft, but if I want to go for a blended one i need to be sure it has this zinc additive. Also 10w40 should be ok as it should have those additives. Please correct me if I'm wrong.


On a side note it's interesting to know 10w40/15w40 are considered diesel oils in USA, while in the EU it wasn't unusual some years ago to use them also on gasoline vehicles (and clearly marketed for such engines).

 

Any oil meeting the recommended viscosity is fine. I pulled my cam out around 100k with using whatever 10w30 was available at the time and it measured to spec. The ZDDP won't hurt but with how mild the stock profile is, it really isn't necessary. The majority of failed cams I have seen are because of cheaper materials in New cams vs old ones failing because of an oil change.

Personally I'd use whatever weight is reccomended and add the zddp back in or some high pressure additive. The diesel comes reccomended from my builder because it still has adequate Zinc and it costs... 12$? A gallon here. Vs 20 or 25$ for 5 quarts of other oils. Use a reccomended grade.

 

Hello again guys, I was about to purchase Mobil 1 ESP with 900 ppm zddp, but then i found many recommendations on the forum about not using this oil on aged engines, as its cleaning properties may remove the deposit which are acting as a glue for gaskets and other old parts, causing the engine to leak oil.
I'm quite confused right now, and the limited amount of products available in the EU doesn't help.
Should i go for dino, blended or synthetic oil with at least 800 zddp ppm? Any product recommendation for my 86 L98 with 79k miles?

Thank you again!

 

That Mobil ESP you considered is just fine. If oil cleaning off gunk in the engine causes a failure(highly unlikely), it's days were numbered anyways. 79k miles isn't even high mileage, I wouldn't worry about it.

 

If you can source this product it will work with ANY oil, plus provides such protection that you can use a grade lighter viscosity also - if you'd like.
https://www.hyperlube.com/product/zi...ment-additive/

 

I wouldn't worry about ZDDP unless you have a flat tappet cam.

I run Motorkote and 15w40 in my engines (newer LS engines though). I seen a back to back dyno run power increase with Motorkote of 10 HP, but I use it more for the lubricity and protection than I do anything else.
If you're not in extreme heat, I would make sure to buy the highest quality 5w30 or 10w30 I could.
Oils today are significantly better than they were in 86. Even the budget non-synthetic oil now is better than what GM ever assumed you would be running in these engines.

 

Thanks JoeNova. i have an 86 L98, which seems to have flat tappet cam according to the information i have, that's why i'm worried about ZDDP quantities.

 

Valvoline VR-1 has a fair amount ,Mobil 15/50 if you like syn does also
No need for diesel oil in an L98 that is so overplayed

 

Use Hy-per Lube Zinc replacement,
https://www.hyperlube.com/product/zi...ment-additive/
, it's the real deal ZDDP replacement additive (Polymer Ester, an expensive synthetic anti-wear additive).
It is not snake oil, but don't take my word for it - go to www.BobIsTheOilGuy.com
and do your own research. Look in particular for a thread in 2015 that includes comments by "MolaKule", he's one of the resident blending tribologists.
Use it with a low ZDDP oil, it also will benefit from a small boost of Moly too. $.02

 

Thanks for the Hy-per Lube Zinc replacement suggestion, unfortunately it seems to be very hard to find in the EU, but i'll keep looking.
May i kindly ask you when you talk "low ZDDP" what value do you mean?

 

All the recent modern oils, API ; SN, SM, SP, etc..., modern regular passenger car oils - and/or any oil not meeting the spec required for an aggressive flat tappet cam & lifter setup.
Fuchs makes some very robust oils if you can find any , but check the composition to be sure it is carrying enough phosphorus & zinc (ZDDP).

 

If you for sure never want to worry about it, there is a motul formulation in a 10w30 flavor that is what the car should need. I'd imagine that would be easier to get over there than here. It is a blend.

 

Check out the links in the first post of this thread:

https://www.corvetteforum.com/forums...corvettes.html

You'll learn a lot about engine oils.

Live well,

SJW

 

Thank you for the suggestion! I believe it's the classic nineties 10W-30 (here attached the spec sheets), which is recommended for 85-91 vettes by the MOTUL oil selector tool, and is reported to have >1800 ZDDP. It's not that difficult to find over there, so i believe it would be a good choice.

 

This one gave me some very useful insights, thank you! It seems that (as everything) too much ZDDP is not a good thing.

 

You may find a product sold within the EU by the name of KetJenLube 135 (or something similar, and made by perhaps an Italian Co) , this may or may not be the same as the product within HyPer Zinc replacement (ie..a polymer ester). Further research would be required.
Good luck, and please update with any findings.

HAPPY HOLIDAYS !!