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Discussion Starter #1
I've just learned that the oil in my 99 S10 hasn't been changed in possibly 11K miles and in addition 3-4 years.

My thought was to put some mobil1 high mileage full synthetic 10W-30 in it since it'll be thicker and might flow better. I doubt this car has had full synthetic before.
 

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Go for it
It will not hurt the engine
 
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The high mileage oils have seal conditioners in them, which cause seals to soften and swell, which for seals that don't need it, isn't good as it just shortens their life as it causes the seal to expand and press harder against the moving surface (e.g. valve stem).

If you don't have oil burning issues or losing oil between changes or drips on the driveway etc. I wouldn't run high mileage oil. Just run regular 10w 30 full synthetic, mobil 1 is fine as are many others.

The high mileage oils were and are more for the last-leg clunkers when you can't afford a new vehicle and need to get another 20-50k miles out of the current one. But it isn't ideal for long term use in classic or truck you intend to keep for a long time.
 

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I've been running full synthetic in my 2.2L going on 14 years now, no issue. I would advise you put the recommended 5w-30 in though. 10w-30 is going to flow worse on cold startup.
 

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Discussion Starter #5
The high mileage oils have seal conditioners in them, which cause seals to soften and swell, which for seals that don't need it, isn't good as it just shortens their life as it causes the seal to expand and press harder against the moving surface (e.g. valve stem).

If you don't have oil burning issues or losing oil between changes or drips on the driveway etc. I wouldn't run high mileage oil. Just run regular 10w 30 full synthetic, mobil 1 is fine as are many others.

The high mileage oils were and are more for the last-leg clunkers when you can't afford a new vehicle and need to get another 20-50k miles out of the current one. But it isn't ideal for long term use in classic or truck you intend to keep for a long time.
Crap, wish I didn't get impatient. I ended up putting 5W-30 synthetic blend high mileage oil in it since the manual doesn't really specify the type of 5W-30 to use under normal conditions.

Next go round, I'll definitely shoot for Mobil1 full synthetic 5W-30.
 

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@manbearpig has a good point. Synthetics offer vastly better lubricity than conventional oils. There really isn't a need for 10w-30 anymore. I haven't run or bought 10w in anything in probably a decade or more now.

If you still want a slightly thicker oil than 5w-30, consider 5w-40 next time around. Good cold start characteristics, bit better in high heat/higher mileage engines. Although staying with 5w-30 would be perfectly fine as well.
 

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If it's had a history of long periods between changes, there may be a sludge build-up. My mechanic told me to add a quart of transmission fluid and let the engine idle for about a half hour, then drive normally for a few miles and drain and change. Transmission fluid is high detergent and can help loosen some of that crap and help you get it out of the crankcase. It's always come out pretty dirty when I did this but I don't have any A:B testing, so take it with a grain of salt.
 
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