Do you think my car would benefit by switching to full syn oil with 215K miles already on it? I asked a couple of friends and they think that it would be a waste of money, and possibly start some leaks. But if I switch I could go a little longer between changes? Just wanted to run it past this forum. thanks

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide