I once switched from 20/50 to 10/40 oil in my V12, I had to actually adjust idle speed because it went higher by about 50rpm. (My oil pressure was fine so i left it until next oil change)
Why do our V12s require such a thick oil?
Clearances. These engines were built with the available materials and manufacturers capabilities available at the time. They don’t have the tighter tolerances that current engines have.
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To add to this, the Main Journals are near 3" in diameter. The generalized 0.001" per inch of diameter clearance gives us near the Jag normal 0.003" main clearances which calls for 20 W50 oil:
And while the rods can be replaced with different ones and undersized journals cut down to a smaller size, that’s not so easy with the Mains… so you’re kind of stuck… Clearances probably COULD be tightened up on the mains a bit (done perhaps while grinding for 10 thousandths oversized bearings) so that a lighter weight more fuel efficient oil could be used, but you’d HAVE to ensure you ran ONLY synthetic (to ensure there was no temp breakdown) and be darned sure the oil was well cooled.
~Paul K.
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That’s an excellent article, Paul: I will stash it for future reference.
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If a person wants to use full synthetic oil in their Jag V12 I would think the oil BMW specs out for the older M5 E60 V10 would be all you would ever need…10W60
I use Mobil1 15W/50 synthetic.
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