I have an 87 300D with the 603.960 engine. This engine has hydraulic lash adjusters to take up slack in the valvetrain automatically. The downside is that, partially due to a design flaw, the lifters get plugged and don't fill up correctly, resulting in nasty ticking noises.
Per Marshall Booth's advice I finally switched to Mobil 1 15w-50 at 191k miles. I had previously switched to Delo 15-40 from cheap Pennzoil 20w-50 in an attempt to flush out whatever junk was causing the lifter noise, as shops were quoting me anything from $25 to $50 PER LIFTER to change all 12 lifters. Delo 15w-40, even with its advanced detergents, did NOTHING to alleviate the noise.
Mobil 1 flowed very quickly as soon as I started the engine with the new oil. Now my car starts more quickly on cool mornings and idles more smoothly under all conditions, and after only 1000 miles on Mobil 1 the valvetrain is remarkably quiet.
Does it leak? Yes, it does, from existing points of leakage. But it doesn't leak appreciably more than the mineral oil did. The engine has "consumed" (leaked or burned) maybe 1/2 a quart in the first 1000 miles, and I think that was my fault anyway; my 603 will leak ANY oil until it gets to a halfway point on the dipstick, then it stops leaking altogether.
I'm confident that I will make it 3000 miles without adding any Mobil 1. For the extra peace of mind it gives me, it is worth the $4.50 a quart. Even at that price, I plan on changing every 3k miles.
[This message has been edited by jrd (edited 06-11-2000).]