what type of oil should I use?

[ScoutDummy]

I have a '79 terra with a 345. I don't know how many miles are on it (speedo didn't work when I bought it), but I'd guess 200k+. I'm currently using Castrol 10W40 in it (a mechanic friend is a Castrol lover). It's had several owners and has had everything from Penzoil 10W30 to Mobil 1 synthetic in it. I usually drive it about 30 minutes each weekend (anywhere from 30 mph in a neighborhood to 55 on open road-- T19 manual transmission, 3.54 gear ratio). The seals are good enough that the dipstick still shows full and the oil looks clean (the Castrol was put in last October). I need to change it, but I'm sort of stuck not knowing the best oil to use. I've read a lot of stuff about oil on the web. Looks like noone agrees but everyone has a very strong opinion. Don't use Quaker State or Penzoil. Definitely don't use synthetic. Or, it doesn't matter what you use. Or, synthetic ensures the least wear-- use it for everything. I'd like to know what you experienced guys think. Thanks.

[Allan E.]

Just keep doing what you have been doing. The OEM spec is straight 30 wt, but keep in mind, that was the 60s and 70s. 10-30 or 10-40 should be fine for most areas. If it doesn't get very hot, stick with 10-30. If it gets hot where you live, use 10-40.

Elsewhere in the vehicle:

Use straight 50 wt motor oil in the t-case. DO NOT USE GEAR LUBE. GL5 eats yellow metals. 50 wt motor oil has the same viscosity as 90 wt gear oil. If you choose to use gear oil, again, DO NOT USE THE KIND WITH THE EP ADDITIVES.

Same goes for the manual transmission.

Use the stuff with the EP additives in open diffs, and use the limited slip additive in the limited slips.