Mixing viscosity of synthetic oil?

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Jul 17, 2006
SW Michigan, near Fennville
I have a new "USED" 1996 Buick Park Avenue with 147,000 mile on it. I've run Valvoline synthetic in my other 1993 PA Ultra and my wife's Chevrolet Lumina (1993). Usually 10-30 in the summer. However, I noticed that the oil pressure was around 30 PSI on this car with the new oil change. So... I changed it again with about 200 miles on it and put in 1 qt of 20-40 Valvoline Synthetic and the rest 10-30 Valvoline Synthetic. Now the pressure is around 45-50 lbs. It comes up to pressure very fast after starting and continues to hold pressure very well.

Has anyone ever heard of this causing problems? I am really considering getting some 10-40 Amsoil from my neighbor up the road.

Also, (and this may be a Buick PA issue) my light comes on saying "check oil level". I do and it completetly full, CLEAN, but full Any idea why that might happen?
You have increased the idle oil pressure, and the drag/load of the oil pump on the engine.
30 lbs at idle is more than sufficient.
I got me one of those LeSabres with the 3800 V6 ( same motor in your Park Ave ) running 10W-40 MaxLife. Oil pressure is 53psi hot. I highly recommend this oil for 3800 V6's over 75K miles.

p.s. My science is based on sound opinion.
I had good luck running both 10W-30 & 10W-40 MaxLife in my 3800...

BTW, I recall reading on Mobil1's old site ("Ask Mobil1") that they advised that it was fine, but unnecessary, to mix synthetic weights (someone mixed 15W-50 & 10W-30 to get 12.5W-40
Mix away. You might want to run 10w30 and add a quart of Mobil 1 15w50 for the thickening.

If you get 40+ psi at speed (say 2000 rpm) and 15 or better at idle, you are fine.

Your car's service manual should tell the oil pressure spec.
I'd skip the Lucas tranny additive and go with a good tranny fluid. Can't beat Redline for severe service in a tranny.

Not the owner's manual, but the shop manual should have the oil pressure spec.

30 psi idle is excellent. I wouldn't put anything thicker for that. What is the pressure when you are cruising at say 2000 or 2500 rpm?
Waiting to get the owners manual in the mail. Bought one off ebay. This car came with no owner's manual, so will wait a few days.

I also want to find out how much brake fluid it holds as I would like to have it power flushed/refilled with either Valvoline synthetic brake fluid or Amsoil brake fluid. I've heard this is an excellent idea on older cars with high mileage to ensure that your ABS keep functioning well.

The oil Gage has red marks at up to 10 PSI. Then there is a mark for 20, 30, 40, etc. Before it was running above 20PSI but not at 40PSI. I just like to see that idle at at least 35-40 psi. Maybe makes little/no difference, but I don't like it teetering on 10 to 20 PSI either.

After seeing an article on this site about Lucas Oil Stabilizer, I'm very hesitant about adding the ATF formula to my transmission at it's change out soon.
I have a 2000 Buick with the 3.8L SC and was running Amsoil 10w30 but it souonded loud/noisy/clacking sound to me so I am currently running a 50-50 mix of Amsoil 5W30 series 3000 and Amsoil 10W40 so in theory a 7 1/2 W 35. So far the MPG is the same as the 10W30, the response appears the same, oil consumption appears the same but wow, it is so much quieter then the straight 10W30. Unless the UOA on this shows bad results I am going to stick with this blend and see what happens. Only 50,000 on the engine.

Originally posted by TallPaul:
What is the pressure when you are cruising at say 2000 or 2500 rpm?

Right around 55 PSI. At idle it's about 45. Oil pressure comes up nice and quick on cold start up and stays well at idle.

Regarding the clacking of the Park Avenue with the Super Charger: I have a 1993 with 120,000 miles on it and it makes a noise like rocks or loose nuts coming from around the supercharge. No, it's not the bearing (usually!) But the SC in the nosecone has a 3 hole nylon "coupler" and this is EXACTLY what it is called, a COUPLER. It kind of dampens the drive of the supercharge from the belt and when it wears a bit it creates slack when idle and "RATTLES". It's about a $45.00 part and time to just take the nose cone off (8 bolts I believe) and replace it and the SC oil.

Might be good to change the SC oil in any case.

What is the pressure when you are cruising at say 2000 or 2500 rpm?

Right around 55 PSI. At idle it's about 45. Oil pressure comes up nice and quick on cold start up and stays well at idle.

That is excellent! If that is on 10w30 I would not go to anything thicker.

Two of my vehicles run around 17 psi idle and 42 at speed hot. One runs 30-40 idle and 60-65 at speed on 10w30. Spec on all three is 40 to 60/65 psi at 2000 rpm hot.
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