adding STP to full syntheitc oil?

[tfret]

I made the switch this year to full synthetic oil after being pleased with results in my cars. I have a 1984 260HP Mercruiser with 5.7L V8 and QuadraJet carb. Since switching to synthetic I have noticed for the first time considerable oil vapor from the pcv hoses coming into the carb. Bad enough that it is causing the engine to stumble on hole shots and idle rough. Carb cleaner makes it go away for a while. My thoughts are that the synthetic oil is blowing by the cylinders much worse than the conventional oil I've always used before. I'm wondering if adding STP to make the oil heavier would solve the problem?

 

[Fordiesel69]

Re: adding STP to full syntheitc oil?

No it will not. STP motor honey is nothing more than a viscocity index improver. Snake oil if will.....

Download a spec sheet for your synthectic and you dino oil The NOACK volitility reading will show why this is happening. Your syn is probably much higher.

The old 350's like SAE30, SAE40, and 15w40 diesel oil is you do not want to use the mercruisr oils.

 

[Howard Sterndrive]

Re: adding STP to full syntheitc oil?

ditto on straight 30
unless you boat in the winter, I don't see the advantage of synthetic in a boat
I love straight grades. less junk, just lubricant

 

[Fordiesel69]

Re: adding STP to full syntheitc oil?

BTW, in my experiance straight oil is better in terms of reducing blow by, and also better in withstanding volitility.

 

[paultjohnson]

Re: adding STP to full syntheitc oil?

I put full syn in my old 77 cutlass. Smoked like a sumbch. FROM WHAT IVE READ, syn flows so much better it will get past the rings easier and tends to leak more for the same reason. I switched back to dino and the smoking dropped considerably................

 

[HT32BSX115]

Re: adding STP to full syntheitc oil?

I made the switch this year to full synthetic oil after being pleased with results in my cars. I have a 1984 260HP Mercruiser with 5.7L V8 and QuadraJet carb. Since switching to synthetic I have noticed for the first time considerable oil vapor from the pcv hoses coming into the carb. Bad enough that it is causing the engine to stumble on hole shots and idle rough. Carb cleaner makes it go away for a while. My thoughts are that the synthetic oil is blowing by the cylinders much worse than the conventional oil I've always used before. I'm wondering if adding STP to make the oil heavier would solve the problem?

You should really have a look at Mercruiser oil recommendations. NO manufacturer suggests using any "additive" AND Other than 25w-40, they don't recommend using multigrade either.

 

[skargo]

Re: adding STP to full syntheitc oil?

I would switch back to whatever you were using before your troubles started.

 

[Silvertip]

Re: adding STP to full syntheitc oil?

The molecules that make up synthetic oil are much smaller than conventional oil. Therefore, if the engine uses conventional oil (even just a little) it will use more synthetic oil. If the engine leaks oil, it will leak more synthetic oil. If blowby is an issue with regular oil, it will be much worse with synthetic. Using synthetic oil and then adding another additive is not a wallet friendly solution to a non-problem. Use what the manufacturer recommends.