Originally Posted By: bubbatime
You do not run synthetic in these engines. Well you can, but you wont like the results. Mazda does not want you to run synthetics in these engines. Plus the engine is 32 years old and has likely run conventional oil its entire life....
I'd run a stout non-synthetic 10W30 diesel oil. Rotella T or similar. Conventional VR1 would also be a good choice.
This is incorrect. Mazda doesn't want people running
PAO or Ester synthetics, but
Group III synthetics are
A-Okay. Furthermore, there is no risk in switching from using conventional to synthetic - in fact, synthetic oil has more seal swellers than conventional and would reduce any current leakage or burning.
That being said, OP's Mazda could continue to use conventional without issue, or could switch to synthetic without issue, as long as he made sure it wasn't PAO or Ester based. Typically though, those Group IV (PAO) and Group V (Ester) oils cost a lot more than Group III synthetics, so it's probably safe to assume SuperTech synthetic 10w-30 would be alright in his Mazda.
All else aside, my personal recommendation to you, Clubber_Lang, would be to either continue using whatever conventional 10w-30 you like (PYB is my favorite conventional), or, you could step up to running a dual rated synthetic-blend Delo 400 XLE 10w-30:
https://www.amazon.com/Delo-400-10W-30-Synblend-Motor/dp/B00M1Y7JXY/
https://www.walmart.com/ip/Chevron-Synthetic-Blend-10W30-Heavy-Duty-Motor-Oil/35950292
Do know, however, that it's a 50:50 chance of getting either the old API CJ-4/SM version or the new API CK-4/SN version when ordering it online from Amazon or Walmart. Honestly either version would be great for your application, though, so whether you would get the blue or charcoal bottle wouldn't really matter too much.