Mobil 1 says

For the hell of it I used there online form and it says to use 5-30 in every category. Ive been using 10-30 Mobil 1 for 3 years on my very high mileage LT1. This summer should I use 10-30 or 5-30? I live in CT and it can get either very hot or very cold. What grade do you guys run in your high miled LT1? Im afraid maybe the 5-30 might leak or loose oil pressure. Thanks.

I use 5w30 Mobil One in my Z28. It's got 129,000 miles and I have no problems with it.

Link to the form you filled out? I use 10-30 but im sure 5-30 would not be bad.

I've been running the 5W30 for awhile now. However, my neighbor gave me 25 quarts of the Mobil 10W30 synthetic for free so that will be used in my future oil changes. I doubt it really matters. At the dealership, we would use 10W30 in all the cars, excluding diesels of course, even if some of them called for the 5W30.

I use M1 10-30 in my Z28 with 112,000 miles, no issues here.

In an LT1 you might get some ticking noise if you use 5W30. I'd stick w/ 10W30.

For the LT1 GM recommended 5W-30, with the option to switch to 10W-30 if you were not operating in a climate where the ambient temperature would fall below 0*F.

As shown in the chart, SAE 5W-30
is best for your vehicle. However,
you can use SAE 1OW-30 if it’s
going to be 0°F (-18°C) or above.

The 5W will show a lower viscosity at very low temperatures, to assist with circulation on cold engine start. Once both the oils warm up to full engine operating temperature, the viscosity will be virtually identical.

10w30 ever since i bought the car in '03. it now has almost 170k on it. when i changed the valve cover gaskets, the heads were clean as a new car. no sludge deposits at all. i bought the car when i was living in michigan, but now i'm living in texas. no ill effects either way.

Mobil 1 is a slightly thin 5w-30.
If I were to waste my money on Mobil...I would run 10w-30.

i used to run 5w-30 in my car, but it kept bothering me that it was so watery. i experimented with 10w-30 and haven't used 5w-30 since. the engine runs quieter and smoother. LS1 with 78,000.

"Mobil 1 is a slightly thin 5w-30.
If I were to waste my money on Mobil...I would run 10w-30."

Mobil 1 5w-30 is 5w-30 and remains 5w-30 months later while the "heavier" wonder oils need to start life heavier to meet a 5w-30 spec after some use.

Mine has had Valvoline 10W40 ran in it as long as I have known. 212K miles

Mobil 1 5w-30 is 5w-30 and remains 5w-30 months later while the "heavier" wonder oils need to start life heavier to meet a 5w-30 spec after some use.
Still too thin for the LSx platform.

Pennzoil Platinum 10w-30 has shown to be a top of the line performer so far with our motors. This is shown via UOA's.

Ive been told to run 10w30. It's better for high mileage. I've had no issues. 5w30 is better for cold weather, but I don't drive my LT1 in the winter.

I normaly run m1 10-30 without any problems.