: Yet another oil question - mix SAE 30 with 10W-30 ?

scott f
12-13-12, 11:23 PM
As the weather becomes cooler, I'm just about to change the oil in the ol' 94 STS.
I've come across some 10-15 yr old sae30 API SL,SJ,SH,SG motor oil...my question is can I use 1l of this with the reguarlly prescribed 10w30 Castrol GTX as a zzdp backup? Hope this makes sense as API SG/SH is whats on my oil filler cap
If this is a good formula I would use 6l of Castrol GTX 10w30 and 1l of SAE30.
Does this sound good for the ol flat tappets?:hmm:
P.S. 296K km
As I start to think about it...what the hell am I doing?
I've made it this far what's the difference?:alchi:


I bought this '94 STS in 2000 with 153K km and didn't have a clue about this forum... just changed the oil with whatever was on sale and 1 bottle of Lucas oil stabilizer,always overfilled! Guess I did something right:helpless:

12-14-12, 10:29 AM
My understanding is that you do not mix straight weights and multi-grade oil. You can mix straight weights like SAE 10 and SAE 30 or multi-grades like 10W30 and 10W40. Run your straight 30 during the summer if you want to.

12-14-12, 11:03 AM
Before I would run straight 30W in a modern engine, particularly a Northstar or other close-tolerance engine, I would do some talking to big-time engine builders. The multigrades were developed to provide proper, quick cold/cool engine lubrication. I won't even go into cold oil and filter bypass valves.............

As Ranger posted, mixing straights and multigrades is not a good idea - there's a lot more to consider than simply the end viscosity.

Want to spend some money and properly put some extra ZDDP into your oil fill ?? Online a case of JoeGibbsRacing HR-4 synthetic oil and use a quart with the other 6.5. Or look into one of the 10W-30 HD oils from Chevron, Gulf, Pennzoil, Shell - and you may find that they carry an automotive SG or SH rating due to their older ZDDP package level.

Leave the Lucas stuff on the store shelf. Yes, they're a big company, yes they advertise well, yes they have stickers on NASCAR, and no, I wouldn't touch any of it with a 10 foot pole. Same applies to another well-marketed additive/cleaner chain. Ask GM if they recommend additives.